AWhat is the best homemade icing to use on a cookie cake that I can put it a bag and frost with ?

there's no one best one

whatever you like best

Here's a really great one.  It would work well on cookie cakes :)

As posted it does NOT crust but a 'fix' is mentioned at the end:)
A smooth, easy to use buttercream icing for all your cakes.
 

Ingredients

Amount Ingredient 2 cups butter 2 cups shortening 2 pounds powdered sugar 2 Tablespoons flavoring

Directions

2 of Everything Icing directions:

In KA mixer (best) mix together butter and shortening for at least 5 minutes. Add about 1/3 of the sugar; mix 5-10 minutes; add another 1/3 of the sugar, along with the flavoring, mixing 5 minutes; add the remaining sugar and mix 5-10 minutes until smooth.
There is NO liquid needed in this recipe.
Flavorings can be *ANY!* combination you want. What I used mostly is: 1 part vanilla; 1/2 part butter flavoring and 1/4 part almond. A "Part" can be any measure you want. Do you make many, many cakes? Mix it up by the cupfull; OR if for only one cake/icing use teaspoons as the 'part'/measure.
Don't like almond? Use lemon OR orange OR whatever you want. But, please do try it as stated at least once:)
I had a friend who didn't like almond but after trying it once just added an additional 1/4 tsp lemon and was happy w/it.
Want this icing to crust? Reduce either the butter OR the shortening by 1 cup OR reduce *both* by 1/2 cup.