Preheat the oven to 350°F. Thoroughly grease and flour a 9-inch round pan. In a medium bowl, stir together the flour, baking powder, and salt.
In a large mixing bowl, beat the sugar and eggs until well blended, about 2 minutes. Add the oil, milk, and vanilla, beating again until well blended. Add the flour mixture to the liquids, beating again until blended.
Spoon half the batter into the empty bowl that held the flours. Whisk the cocoa powder into one of the bowls.
To make the stripes, alternate adding the two different colors to the well-greased and floured pan. Scoop a small amount of batter (see cook's note) into the center of the pan and then alternate with a scoop of the other batter directly on top of the first. Repeat until all the batter is in the pan. The cake batter will spread to the edge of the pan as you add more.
Bake the cake for 40-45 minutes, until a toothpick inserted into the middle of the cake comes out clean. Let the cake cool slightly in the pan, then remove from the pan and place on a wire rack to finish cooling.
When the cake has cooled completely, make the topping. Pour the heavy cream into a mixing bowl and beat with an electric mixer (with the whisk attachment) until stiff peaks form. Add the powdered sugar and vanilla. Beat again to mix throughout.
Scoop the whipped cream topping onto the cake and smooth across. Serve immediately or refrigerate until ready to serve. Enjoy!
Add the batter to the pan in small amounts. It doesn't matter whether you add it 2 tablespoons or 1/4 cup at a time, as long as you add the SAME AMOUNT each time. If you vary the amounts each time you scoop more batter into the pan, you won't see the defined layers and they may bleed together. Keeping the amounts equal each time you add to the pan helps create more defined stripes.