Step 1
Preheat oven to 350 degrees F (175 degrees C).
Step 2
Grease a 9x13-inch sheet pan.
Step 3
Mash vegetable shortening and sugar together in a large bowl until smooth and creamy; beat in eggs until fully incorporated.
Step 4
Stir buttermilk cocoa powder salt and vanilla extract into the shortening mixture until the liquid ingredients are an even color and texture.
Step 5
Sift flour into the liquid ingredients until combined.
Step 6
Pour half the batter into a separate bowl.
Step 7
Lightly stir red food color into one bowl of batter and green food color into the other bowl of batter.
Step 8
Quickly fold 3/4 teaspoon baking soda and 1 1/2 teaspoon white vinegar into the bowl of red batter; quickly fold 3/4 teaspoon baking soda and 1 1/2 teaspoon white vinegar into the bowl of green batter.
Step 9
Pour the red and green batters into the prepared sheet pan in alternating colors.
Step 9
Bake in the preheated oven until the cake is lightly browned on top and a toothpick inserted in the center comes out clean 25 to 30 minutes. Allow to cool completely before frosting.