Prepare 9 x 13 baking pan with butter and flour or a baking spray.
For the Red Velvet Batter
Cream together vegetable shortening, sugar and eggs.
Make a paste of food coloring and cocoa and add to mixture.
Add salt and flour with buttermilk and vanilla.
Alternately add soda and vinegar; just blend, don’t beat hard.
For the Butter Cake Batter
In a separate bowl, cream together butter, sugar, and vanilla until creamy.
Add eggs one at a time, beating well after each addition.
Add flour and salt, mix well.
For the Red Velvet Marble Cake
Spread red velvet cake batter across bottom of prepared baking pan.
Add dollops of butter cake batter alternately across red velvet cake batter.
Run knife through batter to marbleize.
Bake 45-50 minutes at 350º or until toothpick or cake tester inserted into the center comes out clean. You can follow my tips for how to tell when your cake is done.