Chocolate Yam Marble Cake

Name: Chocolate-Yam Marble Cake
Ingredients: 1/2 cup butter, room temperature; 1 cup sugar; 2 eggs; 2 teaspoons vanilla extract; 1/2 orange, zested and juiced; 1 cup pureed yams or 1 (15-ounce can) yams, drained and pureed; 3 tablespoons sour cream; 2 cups flour plus 2 tablespoons; 1 teaspoon baking powder; 1/2 teaspoon baking soda; Salt; 3 tablespoons unsweetened cocoa; Pinch of cayenne pepper; 3 tablespoons hot water