01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a medium bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt until well blended.
03 - In a large bowl, beat butter and granulated sugar with an electric mixer until light and fluffy, approximately 2-3 minutes.
04 - Beat in egg and vanilla extract until fully incorporated into the butter mixture.
05 - Gradually add dry ingredients to wet mixture, mixing just until combined. Add milk and mix until dough forms a uniform consistency.
06 - Scoop tablespoon-sized balls onto prepared baking sheets, spacing 2 inches apart.
07 - Gently press each dough ball with palm or glass bottom to slightly flatten.
08 - Bake for 9-11 minutes until edges are set but centers remain soft. Avoid overbaking.
09 - Let cookies rest on baking sheet for 5 minutes, then transfer to wire rack to cool completely.
10 - Beat butter until creamy. Gradually incorporate powdered sugar, milk, vanilla, and salt until smooth and fluffy.
11 - Spread frosting generously over cooled cookies. Top with sprinkles if desired.