Preheat oven to 350 degrees. Spray two 6-cup Wilton doughnut pans with cooking spray and set aside.
Melt the butter in the microwave and set aside to cool a bit.
In a large bowl, sift together the flour, cocoa powder, baking powder and baking soda.
Whisk in the sugar and salt into the flour mixture.
In a separate bowl, whisk together the buttermilk, yogurt, vanilla, eggs and melted butter.
Make a well in the center of the dry ingredients and pour in the wet ingredients.
Mix together gently with a spatula or wooden spoon until no streaks of flour remain.
Evenly divide the donut batter among the 12 donut wells. It will be about 3 tablespoons of batter per donut.
Bake on the center rack for 11 - 14 minutes, until donuts are springy.
Remove from oven and allow the donuts to cool in pans about 10 minutes before removing and allowing to cool completely on cooling racks. The tops of the donuts will be quite puffy. You will glazing the bottom halves... the halves that were in the donut pans.