In a large mixing bowl, beat butter with an electric mixer on medium to high for 30 seconds. Add granulated sugar, baking powder and salt. Beat until combined, scraping sides of bowl occasionally. Beat in egg, milk and vanilla until combined. Beat in cocoa powder and as much flour as you can with the mixer. Stir in any remaining flour. Divide dough in half. Cover and chill for 1 hour or until dough is easy to handle.