Here’s a recipe that my mom has given me for icing sugar cookies. It’s great because it’s delicious and spreadable like cake frosting, but hardens after a few hours so the cookies are stackable. You’ll have to forgive me, since the measurements are so casual, but old recipes are like that.
Powdered sugar (maybe a bag’s worth)
Softened butter (can substitute vegetable shortening if you need it to be white)
A few splashes of milk
Fill a bowl with powdered sugar, and put in a half stick of room temperature butter. Add a splash of milk, and mix together until thoroughly combined. It should have about the consistency of store-bought cake frosting. If it is too thick, add some more milk. If it’s too thin, add some more sugar. The main thing is to add enough milk that the cookies will harden after drying (straight up butter and sugar won’t get you there), but not too much milk that they’ll never dry out. Frost your cookies with a butter knife, cake froster, or spatula. Leave out to dry, and after a few hours they should be perfectly stackable.