Prep the Butter and Sugar: In a large bowl, beat the softened butter and granulated sugar together until light and fluffy.
Add the Eggs: Incorporate the eggs one at a time, ensuring to mix well after each addition to achieve a smooth batter.
Mix the Dry Ingredients: In another bowl, whisk together the flour, baking powder, and salt to create your dry mixture.
Combine the Mixtures: Gradually add the dry mixture and whole milk to the batter, alternating between the two, starting and ending with the dry ingredients.
Prepare the Chocolate Batter: Set aside one-third of the batter in a separate bowl and mix in the cocoa powder until well combined.
Create the Marble Effect: Grease a cake mold and pour in the vanilla batter, followed by the chocolate batter. Use a knife to gently swirl the two together for that classic marble look.
Bake: Preheat your oven to 350°F (175°C) and bake your masterpiece for 50-60 minutes, or until a toothpick inserted in the center comes out clean.
Cool and Serve: Allow the cake to cool before removing it from the mold. Slice, serve, and enjoy the wonderful blend of flavors!