Cream the butter in a standing mixer, and add both sugars. Continue to cream until light and fluffy; about 30 seconds. Add both eggs. Mix until well incorporated, and then add vanilla, baking powder, and salt. Give it anther good mix, and then add the flour and cocoa powder. Mix well. 

Using a 2 tablespoon measure, scoop cookie dough onto a nonstick baking sheet and bake for 10 to 11 minutes at 375°F. 

Cool on cookie sheet for five minutes, then transfer to a baking rack to cool completely. 

Make the buttercream by beating the butter and powdered sugar until well incorporated.  Pipe or spread onto a cooled cookie, and sandwich with a second cookie. Repeat with remaining cookies.