Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.
In a large bowl, beat the eggs and sugar until light and fluffy.
Add the milk, oil, and vanilla extract, and mix well.
Gradually add the flour and baking powder, mixing until smooth.
Divide the batter into two bowls. Add cocoa powder to one bowl and mix well.
Pour 3 tablespoons of the vanilla batter into the center of the cake pan. Then, pour 3 tablespoons of the chocolate batter on top of the vanilla batter. Continue this process, alternating between the two batters, until all the batter is used.
Bake for 40 minutes or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.