Start by preheating your oven to 350°F (175°C). Grease and flour a ring cake pan to ensure the cake won’t stick.
In a large bowl, cream the softened butter and sugar together until the mixture is light and fluffy. I typically use an electric mixer for this, but you can mix it by hand as well—just ensure it becomes smooth and creamy.
Add the eggs one at a time, mixing well after each addition. This step ensures the eggs are fully incorporated into the batter.
In another bowl, whisk together the flour, baking powder, and salt. Gradually add this dry mixture into the butter and sugar mixture, alternating with small splashes of milk until everything is combined and the batter is smooth.
In a separate cup, mix the cocoa powder with the boiling water to form a smooth paste. Take half of the batter and stir in the cocoa paste to create a chocolate-flavored portion of the batter.
Now, it’s time to marbled the two batters! Spoon alternating dollops of the vanilla and chocolate batter into the ring pan, and then gently swirl them together with a knife or spatula to create the marble effect.
Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for a few minutes before transferring it to a wire rack to cool completely.
While the cake is cooling, prepare the frosting. In a small saucepan, melt the dark chocolate, milk chocolate, and butter together over low heat, stirring frequently until everything is smooth and glossy.
Once the cake is fully cooled, pour the chocolate frosting over the top, allowing it to drip down the sides for a messy, beautiful look. If you like, you can spread it evenly with a spatula to cover the entire top.
