Preheat your oven to 180°C (350°F) and grease a cake pan to ensure nothing sticks.
In a large mixing bowl, beat the eggs with the sugar until the mixture is light and fluffy.
Blend in the melted butter and milk, mixing until well incorporated.
Gradually add the dry ingredients (flour and baking powder) to the mixture, stirring until combined into a smooth batter.
Divide the batter in half and stir the chocolate powder into one half.
In the greased pan, pour alternating spoonfuls of vanilla and chocolate batters to create a marbled effect.
Use a fork to swirl the batters together gently for the marbling.
Bake for approximately 40 minutes, or until a toothpick inserted in the center comes out clean.
