Whisk together the flour, baking powder, salt, and baking soda. Set aside.

Using an electric mixer on medium speed, beat the butter and sugar until light and fluffy. Add the eggs one at a time, mixing well after each addition. Mix in the vanilla.

Reduce the mixer speed to low. Add about a third of the flour mixture. Then add half of the sour cream. Next, add half of the remaining flour mixture, followed by the remaining sour cream and the milk. Finally, add the remaining flour mixture. Mix just until combined.

Divide the batter roughly in half, making one portion slightly bigger than the other. Add the cocoa powder to the smaller portion, mixing just until combined.

Spread half of the vanilla batter in the prepared pan. Top with the chocolate batter. Then top with the remaining vanilla batter. Tap the pan gently on the countertop, and then use a thin spatula or knife to swirl the batters together gently.

Bake 50 to 60 minutes, or until the cake is golden brown and a pick inserted into the center comes out clean.

Place the pan on a wire rack to cool for 15 minutes. Then remove the cake from the pan and place directly on a wire rack to cool completely.