Preheat oven to 350° F. Grease two 9-inch-round cake pans then dust with flour, tapping out excess flour. Line bottoms of both pans with parchment paper.
Whisk the flour, baking powder and salt in a large bowl. Use an electric mixer to beat the butter and sugar in a large bowl , until light and fluffy (3-4 minutes.) Use fork to fold in eggs one at a time, add vanilla and stir until well incorporated, taking care not to over-mix. Stir in the flour mixture in 3 batches, using a fork or low-speed mixer. Alternate adding the milk and stir just until smooth.
Evenly divide the badder into two bowls. Into one bowl, stir in the cocoa powder. Next, divide the white batter by pouring half in one pan and the remaining half in the other pan. Spread evenly.
Use a large scoop or small measuring cup and dole the chocolate batter as shown in the pictures. Next, take a toothpick and gently swirl to incorporate the two batters in each pan, creating a marbled effect.
Bake the cakes for about 30 minutes or until lightly golden and set on top Insert a toothpick in center and it should come out clean when done. Carefully remove cakes from oven and place on cooling rack for 15 minues and then use a small thin knife to loosen edges of cake around the pans. Invert cakes onto wire cooling rack, remove parchment paper, and allow to completely cool for about 30 minutes. Plate and frost as desired.