Preheat your oven to 400 degrees F and line or grease 24 muffin cups.
In a large bowl, combine butter and vanilla Greek yogurt with an electric mixer until completely combined.
Mix in sugar and brown sugar and beat until fluffy.
Add vanilla and pumpkin, mixing until well combined.
Add eggs one at a time, mixing well after each addition.
In a separate bowl, combine white whole wheat flour, baking soda, baking powder, and salt.
Slowly mix the dry ingredients into the wet ingredients.
Divide the batter equally into two bowls. In one bowl, add dark cocoa powder and buttermilk, mixing gently.
In the other bowl, add cinnamon and pumpkin pie spice.
Scoop a bit of each batter into each muffin cup, filling about 2/3 full. Swirl the batters together if desired.
Bake for 15-17 minutes, or until a toothpick inserted comes out clean.
Remove from oven and allow to cool. These muffins freeze great!
