What ingredient makes the buttercream crust?

Well, I will take a shot at this one. Here is my opinion. An icing that is at least half butter to half shortening, well it sets, but it doesn't actually crust. At least not the way I make it with cream and milk. It gets the texture of a rolled buttercream. Now there is some disagreement as to whether or not meringue powder causes crusting or not, I think it increases it personally. But I think the fact that there is cornstarch in the icing sugar in the U.S. and Canada and that in combination with all shortening and water, gives this crusting effect to some degree. The addition of meringue powder increases the crusting action. Think of it like royal icing to some degree, it will crust but not harden up like Royal, because though there is egg whites, dried which is the meringue powder, the fat stops it from hardening. I find that crusting buttercream gets drier on the surface than does a buttercream made with some butter and cream or/and milk. I also believe that an all shortening recipe with icing sugar and water, tends to dry out easier as the water does tend to evaporate in a shorter period of time. That is just my opinion based on my experiences. To slow the process, I would enclose the cake in an airtight container as soon as it is iced. I do this also with my half butter half shortening recipe and if I have to ice a cake a day or two ahead, I box it immediately and then completely seal the boxed cake in a plastic bag. Or I enclose the cake in a plastic airtight container.
Hugs Squirrelly Cakes

Well you could try using cream and or whole milk as your liquid. But is sounds like what you want is more like a canned frosting texture but a better taste and no, short of trying out the milk and or cream in it, you cannot maintain that texture, it is just what happens when the air gets at it. The taste shouldn't really change though, but then, I don't use artificial flavourings so that may be a factor. I do not understand why you would use a butter flavouring when there is real butter in a recipe, but then I don't like the chemical tastes of artificial vanilla or butter flavourings.
Try the milk or cream and sealing up the cake and see if this doesn't make a difference. Personally I find it does. Also if you add more milk or cream and make the icing a more thinned consistency, it will likely stay creamier longer, but the drawback will be smoothing it won't work as well as it will not set in the same manner.
This recipe is basically the Wilton recipe because the ratio of fat to icing sugar is the same - 4 cups of sugar to 1 cup of fat, with the exception of using water instead of dairy product as your liquid. Not the Wilton class recipe but the other one on the site.
I suspect the artificial flavourings lose some of their flavour the longer they sit or are exposed to the air.
Hugs Squirrelly Cakes

Hmm...I don't know. I haven't had these problems. The taste doesn't change on me...could just be the over-crusting that made it seem different. I remember you only added 3T of warm water. I always add 6 and sometimes more. It's stiff enough for me to do good stars but isn't so stiff that it wears out my hand. You also have to keep the cakes tightly covered especially if they're decorated in stars otherwise...gets crunchy Yuck! All crusting BCs will do that. It's just the drying out process.

When it comes to BCs, you just have to find what works for you. Next time you make it, try adding less powdered sugar.

I've tried all butter recipes, 2/3 butter recipes, all shortening recipes, half butter/half shortening...I took what I liked from each recipe and rolled it into one. I wanted the butter flavor from the all butter recipe but the ease of handling from a half butter, half shortening recipe. I wanted it smooth and creamy like the 2/3 butter recipes but I wanted it to crust like an all shortening recipe. That's how I came to the recipe I use.

The extra butter flavor is to compensate for the 1/2 shortening in the recipe and the warm water makes it smooth and creamy. The ratio of powdered sugar to shortening/butter is what makes it crust.

I hope that helps! Before you know it...you'll have your own recipe that works perfectly for you.

I wasn't trying to critique the recipe either, just trying to find a solution to the problem. You have about half of the amount of liquid you require if you used 3 tbsp. of liquid in this recipe, 3 tbsp. covers about 4-5 cups of icing sugar and you had eight cups of icing sugar.
The butter flavouring is used when you make an all shortening recipe, to give a simulated butter taste. It is a matter of personal choice, but personally I find in any recipe that is half butter, the butter overpowers the shortening as shortening really doesn't have a taste, it has more of a texture than anything. In fact that is why it became popular because it doesn't tranfer taste to the items you cook in it.
Also an additional note, placing a cake in a cardboard box will protect the moisture to some degree, but because it is paper, it will also absorb some of the moisture. That is why a plastic sealed container or storing a boxed cake in plastic helps maintain the moisture for a longer period of time. Water in icings tends to evaporate at a faster pace than milk or cream. It is more noticable in an iced cake that has been frozen and thawed out.
Hugs Squirrelly Cakes

Haha, yes, I tend to use green unscented garbage bags a lot for bagging cakes, for bagging buttercream transfers and also, sometimes for garbage, haha! Yes those will do as will the white kitchen garbage bags as long as they are unscented. If the cake is going out to a customer that wants it ahead of time, I use clear painter's plastic dropsheet or the white bags, just in case it looks tacky.
Hhmn, the thing with butter in icing, is that it is much more susceptible to the heat, it melts at approximately 83 F. So if you add your water and it is more than warm, it may well have caused the butter to melt. Also your hand temperature may be higher than some other folks. This is a problem I have, so I actually find a ratio of 5 cups of icing sugar to 1 cup of combined butter and shortening works best for me. Normally I would say that with only 3 tbsp. water for this amount of icing sugar, you should have had an extremely stiff icing, almost impossible to work with. So it sounds like either your water was too hot or your room temperature or hand temperature caused melting.
I actually beat my butter while it is cold, to try to limit this issue for me. My room temperature is a constant 70F otherwise I am one miserable decorator. I used cold cream and milk too, again in an effort to keep the icing from getting too warm.
You may be one of those folks that actually has to use cool water. I have decorating friends that have to keep dipping their hands in ice water while they work, otherwise their icing becomes a sloppy mess and oozes all over the place. But the way you are describing the stars not forming, sounds like the icing was actually melting.
That is the one disadvantage to using an icing with butter in it, you have to be aware of temperatures. But personally, I find that the taste of the end product makes up for it.
Hugs Squirrelly Cakes

Oh...I can't even imagine what a cake covered in stars made with crusting BC would taste like after a day or so left uncovered. It would be really crunchy! At least your one up on making dried BC flowers You can make roses using crusting BC and let them airdry. Makes them easier to put on the cake plus you can make them ahead of time and store them. You can even dust them or airbrush'em.

I put my cakes in boxes and then cover them with cheap, plain white garbage bags (no draw-strings or it will really look like you put your cake in a garbage bag) or I'll just wrap the top and sides with plastic wrap.