Preheat oven to 350 F. Line a baking sheet with parchment paper or a silicone baking mat.
In a small bowl, sift together the flour, baking powder and baking soda.
Place the butter in the bowl of a stand mixer fitted with a paddle attachment and cream on low speed for one minute to loosen.
Add the sugar and cocoa powder to the butter and beat on medium speed until light and fluffy
.Add the egg and vanilla, beating to combine.
Add half the flour mixture, followed by the milk, followed by the other half of the flour mixture.
Use a large (3 tbsp) cookie scoop to scoop the cookie dough onto the sheet, leaving plenty of room around each cookie as they will spread to roughly 3-inches in diameter.
Bake for 12 minutes until the cookies are puffy and spring back when touched.
Remove cookies to a cooling rack to fully cool.
Meanwhile, make the vanilla buttercream frosting.