I agree, Toba Garrett's vanilla cake recipe is great. It is my favorite yellow cake recipe. You might try these options for chocolate marble cake:
1: make 1/2 recipe of vanilla cake and 1/2 recipe of your favorite chocolate cake. Swirl together. I think this makes the best tasting, best textured marble cake. ( Use a chocolate cake recipe that makes a batter about the same consistency as the vanilla cake batter, not too thin. I like the Deluxe Devil's food cake on the back of the Softasilk box. I make it with Hershey's special dark cocoa. It is my favorite chocolate cake recipe).
2: Make a full yellow cake recipe. Take out 2 cups of batter. Add 4 ounces of melted, cooled bittersweet chocolate. Stir just until combined. Swirl with yellow cake batter
3: Make a full yellow cake recipe. Take out 2 cups of batter. Mix 1/2 cup cocoa powder with 1/3 cup boiling water. (Add a little espresso powder if desired). Mix until smooth and cooled slightly. Mix into batter. Swirl with yellow cake.
Hopefully one of these will work for you!