Simple Marble Cake

Add the milk and vanilla extract and mix until well combined.
Add the remaining dry ingredients and mix until well combined and smooth. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated. Do not over mix the  batter.

Combine the flour, baking powder and salt in a medium sized bowl and set aside.
Add the butter, vegetable oil, white sugar and light brown sugar to a large mixer bowl and beat together until light in color and fluffy, about 3-4 minutes. Do not skimp on the creaming time.

Add the eggs two at a time, mixing until well combined after each. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
Add half of the dry ingredients to the batter and mix until combined.

Set aside 1 1/4 cups of batter, then divided the remaining batter between the two cake pans and set them aside.
Melt the chopped chocolate, then add it to the batter that was set aside and gently fold together to combine.
Drop spoonfuls of batter over the top of the vanilla batter that is already in the cake pans. Use a knife to swirl the chocolate and vanilla batters together, being careful not to over combine them.