Instructions

1. Dry ingredients: Combine flour, salt, and baking powder in a bowl
2. Put butter (room temperature) and sugar in  kitchen aid mixture(This is not an affiliated link. Just what I used. You can use any other mixture tool)
3. Mix butter and sugar until its pale white and fluffy
4. Add eggs one at a time from side of the mixer
5. Put in vanilla extract
6. Slowly add the dry ingredients to batter altering with sour cream
7. Separate half the mixture and in the other half add/mix the cocoa powder
8. In the mean time , preheat oven to 375 degrees F.
9. Spray or use brush to grease a 10 cup cake pan with vegetable oil. Also sprinkle a little bit of all purpose flour in the pan and cover the pan from inside with it. This is to make sure the cake does not stick once done, and comes out easily. But make sure you invert the pan and tap the excess flour out of the pan.
10. Dollop large scoops of cake batter  (Step 6/7) in the pan with alternate scoops of chocolate and white.
11. Then take a knife and drag and swirl the batter to create a marbled effect.
12. Bake for 50-60 minutes. Carefully insert a cake tester (You can use a wooden stick or toothpick) It should come out clean when the cake is done.
13. Cool the cake on a wire rack for about 10-15 minutes before inverting on a tray to cool completely.