MIXTURE FOR BOTH CAKES: Preheat oven to 350˚F. Combine sugars with oil in a stand mixer using the whisk attachment. Add eggs, one at a time mixing thoroughly after each addition. Add vanilla. Split in half: pour half of the mixture into a separate bowl. *Mine was 3 cups, so 1½ cups per bowl.
PUMPKIN CAKE: In one of the sugar/oil mixture bowls, add the pumpkin puree & buttermilk. Whisk until combined. In a separate bowl, combine dry ingredients: flour, pumpkin pie spice, baking soda, baking powder & salt. Add to wet mixture until just incorporated. Mixture will be thick. Set aside.
CHOCOLATE CAKE: In the other sugar/oil mixture bowl, add buttermilk. In a separate bowl combine dry ingredients: flour, cocoa, baking soda, baking powder & salt. Add to wet mixture until just incorporated. *The chocolate cake batter is much thinner than the thick pumpkin cake batter, but that is ok.
TO FILL BUNDT PAN: Grease & flour a bundt pan. (I use shortening & a paper towel, but you can use softened butter as well.) Spoon ⅓ pumpkin batter into the bottom of the bundt pan. Drop spoonfuls of chocolate batter, but don't cover the whole pumpkin layer. Alternate spoonfulls with the rest of both batters. Swirl with a knife.
Bake for 45-55 minutes. Check doneness with a toothpick in both chocolate & pumpkin batters. Let cool in pan for 10 minutes, then remove from pan & cool for another 10 minutes.
CREAM CHEESE ICING: In a bowl with a hand mixer, blend powdered sugar & cream cheese until well blended. Beat in vanilla & milk 1 Tbsp at a time until desired consistency. Pour on warm cake. Let cake cool completely.