1.Get an 8-inch cake pan and spray with cooking spray, set aside. In a small bowl add baking powder and flour and whisk until mixed. Set it aside.
2.In a bowl of an electric mixer with whisk attachment, beat the sugar, eggs, and vanilla until creamy and pale. Add 1/2 cup milk and oil and mix until just combined.
3.In two batches, slowly add the flour mixture to the wet ingredients. Mix on low speed until nicely combined.
4.Mix 1/4 cup of milk with dark cocoa powder in a big bowl until smooth. Pour ⅓ vanilla batter into the mixture. Mix until you’ve got two batters, chocolate and vanilla with similar consistency. If your chocolate batter seems slightly thicker, add more milk until both batters match up.
5.Preheat oven to 320 degrees. Spoon 3 tablespoons of vanilla batter onto the pan’s center. Layer 3 tablespoons of chocolate batter right on top. Keep repeating until you’ve used up all the batter.
6.Grab a knife and draw lines from the outer edge towards the center. Place the cake in your oven for about 45 minutes. To check for doneness poke a toothpick into the middle until it comes out mostly clean.
7.Take it out of the oven. Leave it on a wire rack for 15-18 minutes, then lift it off the pan. Serve and slice!