Preheat your oven to 350°F (175°C). Grease two 9-inch round cake pans and line the bottom with parchment paper.

In a mixing bowl, cream together the softened butter and sugar until light and fluffy (about 2-3 minutes). Add the eggs, one at a time, beating well after each addition. Stir in the vanilla extract.

In another bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, alternating with milk and sour cream. Begin and end with the flour mixture. Mix until just combined.

In a separate bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt. In another bowl, combine the sugar, eggs, milk, vegetable oil, and vanilla extract. Mix until smooth.

Gradually add the dry ingredients to the wet mixture and mix until just combined. Stir in the boiling water (the batter will be thin, but this is normal).

To create the half-and-half effect, pour the vanilla cake batter into one side of the prepared cake pans. Then, pour the chocolate cake batter into the other side. Use a butter knife or skewer to swirl the two batters together for a marbled effect.

Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean. Allow the cakes to cool in the pans for 10 minutes, then remove them from the pans and transfer them to a cooling rack.

Once the cakes are completely cooled, you can frost with your favorite frosting (vanilla buttercream, chocolate ganache, or a combination of both). Slice and enjoy!