These are my favorite doughnuts ever. I've had an emotional attachment to them since I was living and working in Washington, D.C. I was super poor and there was a bakery stall in Eastern Market right on my way to work that would sell me two-day-olds for 50 cents, and I'd eat them while riding my bike to work. Oh, the salad days. I'll even eat the 7-Eleven ones in a pinch. I just love a blueberry cake doughnut. I've tried to sneak this doughnut onto the menu of every restaurant I've ever worked at. —Duff
Preheat the oven to 350 degrees F and spray the doughnut pans liberally with cooking spray.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter, sugar, salt, and vanilla until light and fluffy.
In a separate bowl, whisk together the flours, baking soda, and baking powder.
Add the eggs and yolk to the mixer and cream until blended.
In 4 to 6 stages, alternately add the flour mixture and the buttermilk, mixing on medium-low the whole time and constantly scraping down the sides. Once the additions are done, turn the mixer to its lowest setting and gently add the blueberries, mixing just until evenly incorporated and the batter is a nice shade of blue.
Fill a large piping bag with the batter and fill each well in the pans about two-thirds full.
Bake for 18 to 20 minutes or until the dough springs back when poked for regular doughnuts, or 8 to 10 minutes for mini doughnuts.
When cool, dip the doughnuts into the glaze and shake off the excess. Place them on a wire rack so the glaze soaks in and gets a little crunchy on the outside.
Basic Sugar Glaze:
In a big bowl, combine all the ingredients and whisk together. Adjust the consistency by adding either milk (to thin) or powdered sugar (to thicken).