Make the Marble Cake

1. Prepare two 9 inch cake pans with parchment paper circles in the bottom and grease the sides. Preheat oven to 350°F (176°C).
2. Combine the flour, baking powder and salt in a medium sized bowl and set aside.
3. Add the butter, vegetable oil and 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.
4. 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.
5. Add half of the dry ingredients to the batter and mix until combined.
6. Add the milk and vanilla extract and mix until well combined.
7. 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.
8. Set aside 1 1/4 cups of batter, then divide the remaining batter between the two cake pans and set them aside.
9. Melt the chopped chocolate, then add it to the batter that was set aside and gently fold together to combine.
10. 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. I ran a knife in a swirl through the cake in one direction once, then the opposite direction once and that was it.
11. Bake for 18-22 minutes, or until a toothpick inserted comes out clean.
12. Remove the cakes from the oven and allow to cool for about 2-3 minutes, then remove to cooling racks to cool completely.

Make the Frosting

1. To make the frosting, beat the butter and shortening together until smooth.
2. Add half of the powdered sugar and mix until smooth.
3. Add the vanilla extract, cocoa powder and 4-5 tablespoons of water or milk and mix until smooth.
4. Slowly add the remaining powdered sugar and mix until smooth. Add additional water or milk, as needed to get the right consistency of frosting.
5. To put the cake together, use a large serrated knife to remove the domes from the top of the cakes so that they’re flat. These cakes don’t have a large dome, but I like to make sure they’re completely flat.
6. Place the first cake on a serving plate or a cardboard cake round.
7. Spread about 1 cup of frosting evenly on top of the cake.
8. Add the second layer of cake on top, then frost the outside of the cake.