Preheat and prep: Heat the oven to 350°F (175°C). Lightly grease a 6- or 12-cavity donut pan with nonstick spray or a thin coat of butter.

Mix dry ingredients: In a large bowl, whisk 1 1/2 cups flour, 1/2 cup sugar, 1 1/2 teaspoons baking powder, 1/4 teaspoon salt, and 1/4 teaspoon cinnamon until well combined.

Combine wet ingredients: In a separate bowl or measuring cup, whisk 1/2 cup buttermilk, 1 large egg, 3 tablespoons melted unsalted butter (slightly cooled), and 1 teaspoon vanilla.

Make the batter: Pour the wet mixture into the dry ingredients and stir gently until just combined. Stop when you no longer see dry streaks. The batter will be thick but scoopable.

Fold in blueberries: Toss 3/4 cup blueberries with 1 teaspoon flour to help prevent sinking, then fold them into the batter with a spatula. Don’t overmix or the batter may turn purple and dense.

Fill the pan: Spoon the batter into a piping bag or a zip-top bag and snip a corner. Pipe the batter into donut wells, filling each about 3/4 full. Alternatively, use two spoons to portion evenly.

Bake: Bake 9–12 minutes, or until the tops spring back lightly and a toothpick comes out clean. Edges should be lightly golden.

Cool: Let donuts rest in the pan for 5 minutes, then transfer to a wire rack to cool completely before glazing. Warm donuts will thin the glaze.

Make the glaze: In a bowl, whisk 1 cup powdered sugar with 2–3 tablespoons milk and 1/2 teaspoon vanilla. For brightness, add 1 teaspoon lemon juice or a pinch of zest. The glaze should be pourable but cling to a spoon.

Glaze the donuts: Dip the cooled donuts, top side down, into the glaze. Lift, let excess drip off, and set back on the rack. For a thicker coating, let the first layer set 5 minutes, then dip again.

Finish and serve: Let the glaze set for 10–15 minutes until it looks glossy and slightly firm. Enjoy the donuts fresh for the best texture.