This follows largely the recipe at https://www.tastingtable.com/cook/recipes/chocolate-sandwich-cookie-recipe
Make a cookie dough of your choice. Probably about 1/3rd less sugar/butter than would normally be used, and only the egg white - we're aiming for a hard cookie.
Roll out on sheets of parchment paper thin - Using thin chopsticks as guides can help get an even height.
Freeze for an hour. Then use cookie cutters on the frozen dough to get clean edges on a tray to bake.
Bake for 10 min at 350, rotating the pan at 5 min to bake evenly.
For cream, combine 6tbsp butter, 1.25 cups confectionary sugar, and 2tsp vanilla in the mixer until it's turned into butter cream!