For the cake

1. Preheat the oven to 350°F. Prepare two 8-inch round baking pans, line the bottom with parchment paper, and grease the sides. Wrap with bake even stripes if desired.
2. In a microwave-safe bowl, melt the chocolate in the microwave at 50% power in 30-second increments and stir until completely smooth. Allow to cool.
3. In a large mixing bowl, combine the sugar, vegetable oil, melted butter, eggs, egg yolks, and vanilla extract. Beat on medium speed until the eggs and oil are well incorporated and the batter is lighter in color. Next mix in the sour cream and beat until well combined.
4. In a separate bowl, combine the dry ingredients. Alternate adding half the dry ingredients followed by half of the milk and mix just until the flour starts to incorporate. Repeat with remaining ingredients. Use a spatula to scrape down the sides of the bowl and stir the batter from the bottom to the top to ensure it’s well mixed.
5. Measure out 2 ½ cups of the prepared batter into a second bowl. Stir in the melted chocolate until well combined.
6. Alternate pouring the vanilla cake batter into the pan with the chocolate cake batter and gently swirl with a knife. Try to divide the batter evenly between the pans, about 2 ½ cups (combined) of batter per pan. Bake at 350°F for 30-35 minutes. Test the cake for doneness by inserting a toothpick into the center of the cake. If the toothpick comes out clean, your cake is done. Remove from the oven and allow to cool completely.

For the frosting

1. Cut the butter into pieces (about 1 tablespoon in size). Using the paddle attachment, whip butter for 5-7 minutes, scraping down the bowl occasionally. Beat until the butter is really light and fluffy. Next, beat the cocoa powder into the butter until well-mixed.
2. Alternate adding 2 cups of powdered sugar at a time with the liquid ingredients: vanilla extract & heavy cream. Ensure the powdered sugar is well combined, and whip for 1-2 minutes before adding more powdered sugar. Once all the powdered sugar has been added, increase the speed to medium-high and beat for another 2-3 minutes to whip additional air into the frosting.

To assemble the cake

1. Level off the cakes by removing any domes. Place a dollop of frosting on your cake board and place the bottom layer of cake centered on the cake board.
2. Pipe a layer of frosting in circles working outside edge to the middle. Spread evenly with an offset spatula. Add the second layer of cake, inverted. Apply a crumb coat the cake and put in the freezer for 5-10 minutes. Remove from freezer and add remaining frosting to the side and top of the cake. Decorate and serve.