Place your 1 light colored cake batter and 1 dark colored cake batter next to your prepared baking pan on a stable surface.

Using a large spoon, scoop 1/3 of the light colored cake batter into the pan in a down–up–down pattern. If you are using a small loaf pan, place three large dollops; if you are using a round tube or bundt pan, drop dollops in the same down–up–down pattern all the way around the pan.

Scoop 1/3 of the dark colored cake batter into the empty spaces to form a rough checkerboard pattern on the base of the pan.

For the second (middle) layer, use the next 1/3 of the batters but reverse the order so the colors alternate vertically: first scoop the next 1/3 of the dark colored batter into the down–up–down positions above the light squares below, then fill the remaining spaces with the next 1/3 of the light colored batter to complete the middle checkerboard layer.

Repeat the pattern for the third (top) layer with the remaining batter: scoop the final 1/3 of the light colored batter into the same positions you used for the bottom layer, then fill the remaining spaces with the final 1/3 of the dark colored batter. You should now have three checkerboard layers.

Take a butter knife and gently swirl: make rounded vertical zig-zags from one side of the pan to the other. Wipe excess batter from the knife, then repeat the same zig-zags in the opposite direction. If using a loaf pan, make long horizontal swirls instead; if using a round tube or bundt pan, make opposing zig-zags all the way around to create a chain-shaped swirl. Do not over-swirl—fewer swirls yield a clearer marbled pattern.

Bake the cake according to your cake recipe's baking instructions. Allow the cake to cool completely before slicing to reveal the marbled interior.