How to Make It
Preheat oven to 350°F.
Weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine flour and next 4 ingredients (through baking soda) in a medium bowl, stirring with a whisk.
Combine granulated sugar, oil, sour cream, rind, and eggs in a large bowl, stirring with a whisk. Stir in pumpkin. Gradually add flour mixture, stirring just until moist. Transfer 2 cups batter to a medium bowl; stir in chocolate and vanilla.
Spoon chocolate batter alternately with plain batter into a 10-cup Bundt pan coated with cooking spray. Pull a thin knife through batter to swirl. Bake at 350°F for 50 minutes or until a wooden pick inserted in center comes out clean. Cool 15 minutes. Remove from pan; cool completely on a wire rack. Sprinkle with powdered sugar.