Let me say up front that this is one of the finickiest cookie doughs I've ever dealt with. In order to get that dry crunchy chocolate cookie to sandwich the rich creamy frosting, you must be prepared for seriously crumbly batter. But if you persevere, good things will happen.
1 1/4 all purpose flour
1/2 cup unsweetened cocoa
1 tsp baking soda
1/4 tsp baking powder
1/4 tsp salt
1 1/2 cups granulated sugar
1 1/4 sticks room-temperature butter
1 large egg + 1 egg yolk
Frosting (I used the same buttercream I use to frost cookie cakes)
1. Set two racks in the middle of the oven. Preheat to 375 F. Line baking sheets with parchment.
2. Cream together the butter and sugar. Add egg and yolk.
3. In another bowl, combine flour, cocoa, baking soda, baking powder and salt. Mix these dry ingredients into the wet ones.
4. Use your hands to mold balls of dough and drop evenly onto the cookie sheets, leaving a few inches in between. Bake 9 minutes (don't overdo it or they'll be impossible to chew!).
5. Sandwich the cookies with PLENTY of frosting.