If you are going to use real whipped cream, I find that it holds up best using the Wilton Stabilized Whipped Cream recipe. You can also make another recipe using gelatine or you can add a package of Dr Oetker Whip-it but from my comparison tests and in my opinion, I still prefer the Wilton one. You have to be extremely careful not to overbeat your cream, it will not feel like buttercream when it is stiff enough, in fact it won't feel like you will be able to pipe with it, but you will. Stiff peaks is when you put a spoon in you can draw it to a peak and the peaks will not tip over, soft peak means the whipping cream holds its shape to a degree but the tips will fall over. Yes you must keep whipped cream iced cakes refrigerated but with using the Wilton recipe, it will keep well in a fridge for 3 or 4 days. I found using other methods of stabilizing, the cream started to separate after a day or so. You can pipe shells or starts but it is not meant for detailed work like basketweave and such.
I also am copying the Whimsical Bakehouse Buttercream recipe. I do find it tastes better with high ratio shortening, I personally like it, it isn't sweet, it does have a bit of an odd texture but I am fine with it. Not everyone's cup of tea, my family doesn't like it. Do no refrigerate it or it goes hard but you can return it to room temperature and re-whip.
Whimsical Bakehouse Buttercream
In the bowl of an electric mixer,Stir together
6 cups of Confectioners Sugar
1/2 teaspoon salt (I prefer to add the salt to the boiling water below and not mix it in here)
1 teaspoon Vanilla Extract
With a whisk attachment,add and whip at low speed
1 cup of boiling water(3/4 cup on hot days) (I add the salt here)
Whip until smooth and cool
Add and Whip until smooth
2 3/4 cup high ratio shortening or regular vegetable shortening
6 ounces( 1 1/2 sticks of butter) slightly chilled cut into small inch
pieces. If you use salted butter do not add the salt called for in the recipe.
Increase the speed to med-high and whip until light and fluffy and it
will have doubled in size (10-20 minutes - I find 10 minutes adequate)The buttercream will almost fill a 5 quart mixing bowl.
Yields 9 1/2 cups
Please note, this is not a crusting buttercream so you cannot use the Viva Paper Towel method of smoothing.
Wilton Stabilized Whipped Cream Icing
1/2 pint (1 cup) heavy whipping cream
2 tablespoons confectioners' sugar
2 tablespoons Wilton Piping Gel
1/2 teaspoon Wilton Clear Vanilla
Combine whipping cream and sugar in mixing bowl. Whip to soft peak stage. Add piping gel and vanilla, then continue to whip stiff peaks. Do not overbeat.
YIELD: 1 1/2 to 2 cups.
As an alternative, you can use Frozen Non-Dairy Whipped Topping or Packaged Topping Mix. Thaw frozen whipped topping in refrigerator before coloring or using for decorating. Use packaged topping mix immediately after preparing. Do not allow either to stay at room temperature, as it becomes too soft for decorating. Store decorated cake in refrigerator until ready to serve.