Step 1: Preheat your oven to 350°F (175°C). Generously grease and flour a 10-inch Bundt pan or a standard loaf pan to ensure the pumpkin chocolate swirl cake releases easily.
Step 2: In a medium bowl, whisk together the flour, baking soda, baking powder, and salt. Set aside.
Step 3: In a large bowl or stand mixer, cream the softened butter and sugar together on medium-high speed until the mixture is light and fluffy, about 3-5 minutes.
Step 4: Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract and sour cream until well combined.
Step 5: Gradually add the dry flour mixture to the wet ingredients on low speed, mixing just until combined. Do not overmix.
Step 6: Divide the batter into two equal portions. To one portion, stir in the pumpkin puree, cinnamon, nutmeg, and cloves. This is your pumpkin base.
Step 7: To the second portion, fold in the cocoa powder, milk, and melted chocolate. This is your chocolate base for the pumpkin chocolate swirl cake.
Step 8: Alternately spoon the pumpkin and chocolate batters into the prepared pan. Use a knife to swirl the batters together gently to create a marble effect.
Step 9: Bake for 55 to 65 minutes, or until a wooden skewer inserted into the center comes out clean. Cool the pumpkin chocolate swirl cake in the pan for 20 minutes before inverting onto a wire rack.