Set oven to 350º F. In a small bowl, rub two tablespoons of butter together with 3 tablespoons of flour until it becomes a paste. Rub the paste into every crevice of the bundt pan possible. Make sure the entire pan is covered. This will help your bundt cake to come out in one piece.
In a medium sized bowl, add the flour, baking soda, baking powder, salt, and cocoa powder. Whisk together until homogeneous and no clumps of cocoa powder remain. Set aside.
In a large bowl, whisk together sour cream, vegetable oil, and mayo. Whisk until homogenous, then add the sugar, eggs, and vanilla. Whisk again until homogeneous. FInally, add the milk, slowly, whisking constantly, making sure every ingredient is evenly distributed.
Add the dry ingredients to the wet ingredients. Whisk together until just combined and no streaks of flour remain. Pour into prepared bundt pan.
Bake in preheated oven for 50 minutes or until a toothpick inserted in the center comes out clean. Allow to cool for 20 minutes before attempting to remove from pan. Be gentle, patient, and careful as the cake will still be hot at this time.