01 -
Combine butter and both sugars until fluffy, about 3-5 minutes. Stir in the egg. Add the flour, cocoa, salt, and baking soda until dough forms.
02 -
Cut dough into two portions. Flatten between sheets of parchment to 1/8-inch thickness. Chill for 30-40 minutes.
03 -
Use a 1 1/2-inch cutter to shape circles. Arrange 1 inch apart on lined trays. Bake at 350°F for 12-14 minutes until firm.
04 -
Whip butter until creamy. Slowly mix in powdered sugar, then add milk, vanilla, and salt until fluffy.
05 -
Apply filling onto the flat side of one cookie, then cover with another cookie, flat sides together.