1) Preheat the oven to 350°F. Lightly spray a 9" x 2" round baking pan with cooking spray. Line the bottom of the pan with a circle of parchment, and spritz again. Set the pan aside.
2) In the bowl of your mixer, blend the sugar and eggs until lightened, about 2 minutes. On low speed blend in the oil, milk and vanilla until well combined and smooth.
3) In a separate bowl, combine the flour, baking powder and salt. Whisk to remove any lumps. Add the dry mixture to the wet ingredients. Combine on medium speed 1-2 minutes or until the batter is smooth and lump free. Be sure to scrape down the bowl halfway through mixing.
4) Remove 2 cups of the vanilla batter and place it in the measure you used for the liquid ingredients. Sift the cocoa over this batter, and stir well to combine. Be sure to use a sifter to avoid cocoa lumps in the batter.
5) Now for the stripes. Spoon about 3 tablespoons of vanilla batter into the center of the cake pan. Next, spoon 3 tablespoons of the chocolate batter into the center of the vanilla batter. This causes the vanilla batter to spread out. Continue to alternate batters, in bulls-eye fashion until all batter is used. You will now have thin rings of each batter on the outer edges of the pan, thicker rings towards the center.
6) Bake the cake in the center of the preheated oven for 35-45 minutes, or until the cake is lightly browned and a toothpick inserted into the center comes out clean. Cool the cake in the pan for 10 minutes, and then turn out onto a wire rack to finish cooling. Frost as desired.