Preheat oven to 375°F. Grease and flour the 6 3 1/4-inch cups of a doughnut pan; set aside. In a small bowl whisk together the flour, baking powder, and salt; set aside.
In a medium bowl beat the butter and sugar with an electric mixer on medium speed for 30 seconds. Add the egg, buttermilk, and vanilla. Beat until combined. Add the flour mixture to the beaten mixture and beat until just combined. Fold in the blueberries.
Spoon the batter into the prepared cups, filling each about 3/4 full and spreading the batter evenly. Bake for 15 minutes or until golden. Turn the doughnuts out onto a cooling rack. Cool the pan, wash, dry, and grease and flour 4 of the cups. Fill the prepared cups with the remaining batter and bake as above. Dust doughnuts with powdered sugar before serving, if desired.