In the bowl of a stand mixer combine sugar, brown sugar, flour, cocoa powder, baking soda, baking powder and salt.
In a separate bowl whisk together egg, egg yolk, sour cream, milk, oil and vanilla extract.
Pour the batter into the prepared pan. Bake in preheated oven for 30 – 35 minutes, or until a wooden toothpick inserted in the center of the cake comes out clean.
Cream butter on medium-speed until completely smooth, about 3 minutes.
Turn mixer off and add powdered sugar and cocoa.
Turn mixer on lowest speed and mix about 2 minutes.