Using a handheld or an electric mixer fitted with the paddle attachment beat the butter for about a minute until smooth and creamy.
Add the granulated sugar and beat on high speed for 3-4 minutes until the mixture is pale yellow.
Beat in the eggs one at a time then mix in the vanilla extract.
Pour in milk and mix well.
In a large mixing bowl, sift the flour, baking powder, and salt.
Add the dry ingredients, 1 cup at a time and mixing after each addition until just incorporated.
Divide the batter in two equal parts. Divide one of the parts into your prepared baking pans.
Mix the cocoa powder with the other half of the batter using a wooden spoon.
Spoon the chocolate batter equally into the two loafs, on top of the white batter.
With a stick/wooden spoon or a butter knife make waves in the batter so as to create the marbling.
Bake in the oven until done, will take anywhere from 35-50 minutes.
Cake is donw when a toothpick is inserted in the middle comes out clean.
Remove cakes from the oven and allow to cool for 30 minutes in the pan then take it out and let it cool completely.