Basic Marble Cake

This is a basic marble cake recipe I came up with just by altering my go-to, no-fail, classic chocolate cake recipe. The flavor and method works really well as a marble cake, and the result is an incredibly moist, and prettily swirled cake.

Basic Marble Cake
1 3/4 cups all-purpose (plain) flour
2 teaspoons baking soda
1 teaspoon baking powder
1/2 teaspoon salt
2 cups granulated (white) sugar
1 teaspoon instant coffee
1 cup boiling water
2 large eggs
1 cup buttermilk
1/2 cup oil
2 teaspoons vanilla extract
1/4 cup (heaping) unsweetened cocoa powder

Preheat oven to 350◦F. Grease or line a 9x13 inch rectangular baking pan. In a large bowl, sift together the flour, baking soda, baking powder, and salt, then the sugar. Mix well. 
In a separate medium bowl, mix together the coffee and water, then beat in the eggs, milk, oil, and vanilla until frothy. 
Pour wet ingredients into dry ingredients and beat at low speed for thirty seconds to combine. Then beat at medium speed for two minutes. Pour half of batter into prepared pan. 
Sift cocoa into remaining batter and mix well. Slowly pour the chocolate batter in a thin stream over the vanilla batter in the pan, using a back and forth motion and circling around the pan to create a marbling effect. 
Bake for about 35 minutes until a toothpick inserted in the center comes out clean.