Combine butter and yogurt with electric mixer until completely combined. Mix in sugar and beat until fluffy. Add eggs and mix until well combined. Add vanilla and pumpkin and mix until well combined. In a separate bowl, combine white whole wheat flour, cocoa powder, baking soda, baking powder and salt. Slowly mix into the wet ingredients. Add in buttermilk, and mix just until combined. If adding chocolate chips or nuts, fold in.