preheat oven to 350F
cream together 1/2 cup (1 stick) + 2 tbsp softened butter + 3/4 cup granulated sugar + 1 tsp vanilla
whisk in 1 egg just until combined
in another bowl mix together 1 1/3 cup all purpose flour + 1/3 cup & 2 tbsp cocoa powder + 1/2 tsp baking powder
add to wet ingredients and stir all together
scoop on lined baking sheets (24 total cookies)
flatten with palm of hand to make thinner
refrigerate for 15 min
bake for 12-14 min
let cool and make filling:
in a stand mixer, whisk together 1/2 cup softened butter + 2 1/4 cup powdered sugar until fluffy
add evenly to cookie, place another cookie of the same size on top and lightly press together