Preheat the oven to 350°F. Grease a 13x9-inch baking pan with cooking spray.
In a large bowl, whisk together the flour, cocoa powder, baking soda and baking powder with 1/2 teaspoon kosher salt.
In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and sugar until light and fluffy. Beat in the egg 1 egg then beat in the sour cream and vanilla extract until combined.
Alternately add the milk and flour mixture in three increments until combined.
Transfer the batter into the prepared cake pan, spreading it into an even layer.
Bake the cake until a toothpick inserted comes out clean, 18 to 22 minutes. Let the cake cool completely.