Preheat the oven to 350°F (175°C). Grease and flour two round cake pans.
In a medium bowl, mix together the dry ingredients: flour, baking powder, and salt.
In a separate bowl, whisk the eggs, milk, sugar, and melted butter together until smooth.
Divide the batter into two portions. In one portion, mix in cocoa powder for the chocolate batter. In the other, add vanilla extract for the vanilla batter.
Drop spoonfuls of alternating batters (chocolate and vanilla) into the center of the cake pan, creating a circular pattern. Continue layering until the pan is filled.
Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cakes to cool before frosting with buttercream or cream cheese frosting.
Notes
You can make a rainbow version by adding food coloring to each batter or try different flavor variations like adding lemon zest for a citrusy twist.