Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.

In a large bowl, cream the butter and sugar together until light and fluffy.

Add the egg and vanilla extract, mixing until well combined.

In a separate bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture, mixing until a soft dough forms.

Scoop tablespoon-sized amounts of dough, roll into balls, and place on prepared baking sheets about 2 inches apart.

Flatten each cookie slightly with your hand or the bottom of a glass.

Bake for 8–10 minutes, or until cookies are set around the edges but still soft in the middle. Let cool completely before frosting.

To make the frosting, beat the butter until smooth. Slowly mix in powdered sugar, then add vanilla, salt, and milk a little at a time until desired consistency is reached.

Frost the cooled cookies and add sprinkles if desired.