Preheat the oven to 350 degrees Fahrenheit. Grease the pan and sprinkle with sugar.
With a stand mixer, cream the butter, shortening, sugar, and pudding. Combine until fully mixed and smooth.
One at a time, add the eggs into the creamed mixture. Beat the mix together until fully combined.
Add the sour cream and vanilla into the mix. Mix together until fully combined.
Sift the flour, baking powder, and salt together. This will get rid of any lumps and allow for a smooth texture.
Add the sifted ingredients into the wet ingredients, alternating adding the sifted ingredients and milk to the wet ingredients. Do this until fully mixed together.
Once all the ingredients are fully combined, add the cake mix into the pan. Spread the cake until it is even in the pan.
In the preheated oven, bake the cake for 40-50 minutes. To check to see if the cake is done, use a toothpick. If it comes out clean the cake is done, if there is batter or crumbs stuck on it, then the cake needs more time.
Allow the cake to fully cool on a wire rack. Decorate with frosting if desired. Store cake in an airtight container at room temperature if not frosted and in the fridge if frosted.