Preheat oven to 350° with rack in middle of oven. Spray donut pans with nonstick cooking spray; set aside.

In medium bowl, whisk together All-Purpose Flour Artisan Blend, cocoa powder, baking powder, baking soda, and salt; set aside.

In bowl of stand mixer with paddle attachment, beat sugar and eggs together until soft and a lemon color. Add buttermilk, sour cream, and vanilla, and mix well. With mixer on low, slowly add flour mixture until completely mixed.

Put butter and chocolate in microwave-safe bowl, and microwave for 30 seconds. Remove and stir to fully melt chocolate into butter. If needed, microwave another 15 seconds and stir again. Add warm chocolate butter to batter and mix again to incorporate; batter will be thick and fudgy.

Fill large ziplock bag or pastry bag with batter and cut 1-inch opening across one corner of ziplock or tip of pastry bag. Squeeze batter into donut pan molds.

Bake 15 to 18 minutes, until donuts are puffed and spring back when gently touched. Cool in pan a few minutes before carefully removing to wire rack. Allow to cool completely before frosting or dusting with powdered sugar.