Preheat oven to 400 degrees. Line or grease 24 muffin cups. Mix butter and sourcream with electric mixer until completely combined. Mix in sugar and brown sugar and beat until fluffy.
Add vanilla and pumpkin and mix until well combined. Add eggs and mix thoroughly.
In a separate bowl, combine flour, baking soda, baking powder and salt. Slowly mix flour mixture into the wet ingredients.
Divide batter equally into two bowls. Into one bowl add buttermilk and cocoa powder, and mix just until combined. In the other bowl, add cinnamon, ginger, allspice, and nutmeg.
Scoop a bit of each batter into each muffin cup (totaling about 2/3 of the way full with both batters combined). Use a knife to swirl together the chocolate and pumpkin.
Bake for 13-15 minutes, or until a toothpick inserted into the center of a muffin comes out clean.