Using the paddle attachment, beat the butter and the sugar until light and fluffy. About 3-4 minutes.
1 cup Butter, 2½ cups Sugar
Add the eggs, one at a time, and beat after each addition. Add the vanilla extract.
4 Eggs, 1 tablespoon Vanilla extract
Slowly add in the buttermilk and mix well.
1 cup Buttermilk
Add the dry ingredients and gently mix. DO NOT OVERMIX.
How To Make The Chocolate Batter
Place the chocolate chips and the butter in a bowl and place on a double boiler. Stir occasionally until it is melted and mixed.
¼ cup Butter, ½ cup Semi-sweet chocolate chips
Add in the cocoa powder and mix well. Remove from the heat and add in the vanilla extract and let cool down.
1 tablespoon Cocoa powder, ½ teaspoon Vanilla extract
Place two cups of the pain batter in a bowl and add the above chocolate mixture. Add the sour cream and mix. DO NOT OVERMIX.
½ cup Sour cream
How To Assemble
Place one scoop of the plain batter at the bottom of the pan, and using another scoop, place one scoop of the chocolate batter on top of the plain batter. Repeat these until all the batter is used.
Tap the pan on the counter a few times. Bake for 50-60 minutes.
Let cool down for 15 minutes, then flip over a cooling pan, and let cool down completely.
Notes
If using "Cake Flour", just add 3 cups for this recipe.
If you do not have "Cake Flour" available, for each cup of "Cake Flour", remove two tablespoons of the flour and replace it with corn starch.