Preheat oven to 325 degrees F. Line a large baking sheet with parchment paper or nonstick foil.
Combine the flour, cocoa, espresso powder (if using), baking soda, baking powder and salt and set aside.
In a mixing bowl, beat the butter until creamy. Add the sugar and beat until creamy, then beat in the egg and vanilla. Add the flour mixture and stir to make a thick dough.
Using a level teaspoon, scoop up dough and shape into small balls. Set the balls on the lined baking sheet and press down to about ⅛ of an inch with bottom of a glass or the palm of your hand. For perfect circles, after pressing down the dough punch through with a 1 ½ inch cutter and remove the scraps.
Bake circles for about 10 minutes or until cookies appear set. Let cool on the baking sheet for about 3 minutes, then transfer to a wire rack. The cookies will firm up a bit as they cool.
Prepare filling. Beat together butter and confectioners’ sugar until well blended, then add vanilla and milk (or half and half) and beat until creamy, adjusting liquid as needed. Put in a heavy duty freezer bag or a piping bag and snip off the bottom corner. Squeeze a dab of icing in the center of half the cookies and cap with remaining cookies.
Total cookies should be around 40 depending on the size of your cookie cutter and how thin you cut the dough.
Notes
To halve the recipe, weigh out half of each ingredient on a scale and use 22-25 grams of beaten egg.