In an electric mixer fitted with a paddle attachment, cream together the room temperature butter with the white and brown sugar.
½ cup butter, ½ cup brown sugar, ½ cup granulated sugar
Beat in the egg, milk, and vanilla extract.
1 large egg, 2 teaspoons vanilla extract, 2 tablespoons milk
In a large bowl, whisk together the dry ingredients.
2 cups all purpose flour, ½ cup cocoa powder, 1 teaspoon baking powder, ½ teaspoon salt
Gradually add the dry ingredients into the batter. Scrape down the sides of the bowl as needed.
Grease a 9x13 pan and transfer the batter into the prepared pan using a rubber spatula.
Bake at 350F for 12-15 minutes until cooked through. Cool completely before frosting.