Sift flour and baking powder; set aside. In a stand mixer fitted with the whisk attachment, cream butter with salt first, then add vanilla and powdered sugar. Add eggs gradually – mix until homogenized. Set speed on low, and incorporate the sifted flour and baking powder. Meanwhile, mix lukewarm milk with cocoa powder and a third of the vanilla batter ≈ a pound/450g). Grease pan with softened butter and cooking spray. Alternate layer of vanilla and chocolate batter. Run a wooden skewer through batter in a figure-eight motion. Bake right away, or refrigerate cake for up to 3 days.
Preheat oven to 360ºF/180ºC. Bake for 45 minutes or until a knife inserted in the center comes out clean. Unmold and cool on a wire rack. Baked marbled cake can be wrapped and kept refrigerated for up to 5 days. Enjoy!