For the Cake:
Preheat oven to 350 degrees. In a bowl, whisk the flour, baking powder and salt.
In an electric mixer fitted with the whisk attachment, beat the butter on high until creamy, 2 minutes. With the machine on, gradually add the sugar and beat, scraping the bowl, until fluffy, 8 minutes. Beat in the eggs one at a time, scraping the bowl. Beat in the vanilla. At low speed, add the flour mixture in three additions, alternating with the milk. At medium speed, mix until smooth.
Transfer 2 cups of batter to another bowl; stir in cocoa powder. Pour the remaining yellow batter into a foil-lined cake pan. Spoon blobs of cocoa batter on top of the yellow batter. Using a butter knife, swirl in the cocoa blobs. Bake as directed.