Prepare cake mixes as directed on package in two separate bowls.
Wax paper line or lightly grease and flour the sides and bottom of a 9 in. (23 cm) square cake pan.
Drop batter by tablespoons into the pan, alternating chocolate and white batters.
Bake at 350°F (175°C) for 25 to 35 minutes or until tester inserted in middle comes out clean.
Cool in the pan for about 10 minutes; then transfer cake to rack.
Cut cake into 20 servings.