Lightly grease two donut pans with cooking spray and preheat the oven to 350°F (176°C). (I use a paper towel to evenly distribute the spray in each well and to dab off any excess.) This recipe yields 10-12 donuts.
Melt the baker's chocolate in the microwave in a heatproof bowl. Mix the espresso powder with the melted chocolate until completely combined. Set aside to cool slightly.
In a medium bowl, whisk together the flour, cocoa powder, baking soda and salt until combined.
In a large bowl, whisk the sugar, egg, sour cream and vanilla until smooth. Then whisk in the melted chocolate until smooth. Fold in 1/3 of the flour mixture, followed by 1/2 of the buttermilk, another 1/3 of the flour, the remaining buttermilk and the remaining flour.
Spoon the batter into a large plastic bag and cut off the tip, making a hole about an inch wide. Pipe the batter into the greased donut wells, about 3/4 full. I usually get 11 donuts, but you might be able to squeeze out a 12th!
Bake for 8-10 minutes, or until a toothpick inserted in the center comes out clean. Leave the donuts in the pan for 3 minutes before removing to a wire cooling rack.