Step 1: Preheat your oven to 350°F (175°C). Lightly grease a 12-cavity donut pan with non-stick cooking spray or butter.
Step 2: In a large bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt until well combined.
Step 3: In a separate medium bowl, whisk together the buttermilk, melted unsalted butter, large egg, and vanilla extract.
Step 4: Pour the wet ingredients into the dry ingredients. Mix with a spatula until just combined. Be careful not to overmix; a few lumps are fine.
Step 5: Gently fold in the fresh or frozen blueberries. If using frozen, add them directly from the freezer without thawing.
Step 6: Transfer the donut batter into a piping bag or a large Ziploc bag with the corner snipped off. Pipe the batter evenly into each greased donut cavity, filling them about two-thirds full.
Step 7: Bake for 10-12 minutes, or until the donuts are golden brown and spring back when lightly touched. Let cool in the pan for 5 minutes before transferring to a wire rack to cool completely.
Step 8: While the donuts cool, prepare the glaze. In a small bowl, whisk together the powdered sugar, milk (start with 2 tablespoons and add more if needed), and vanilla extract until smooth and pourable.
Step 9: Once the donuts are completely cool, dip the top of each donut into the glaze, allowing any excess to drip off. Place them back on the wire rack to let the glaze set. Serve immediately or store.