In a food processor add the flour, baking powder, salt, butter, sugar, eggs, milk and vanilla. Pulse for approximately 2 minutes until combined.
Separate into 2 bowls, one bowl. Add the cocoa and combine well.
Alternately, spoon the vanilla and chocolate batter into the prepared cake pan, even the top with a spatula and with the tip of a knife swirl it in the batter to make a marble effect.
Bake for approximately 50 minutes, let cool 20 minutes in the pan then remove the cake to a wire rack to cool completely.
Heat butter and cream until hot but not boiling, stir until smooth, let cool to lukewarm and it has thickened then drizzle or pour over the completely cool cake.