Step 1: Preheat your oven to 350°F (175°C). Lightly grease two 6-cavity donut pans and set aside.
Step 2: In a large bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt.
Step 3: In a separate medium bowl, whisk together the beaten egg, buttermilk, melted butter, and vanilla extract until well combined.
Step 4: Pour the wet ingredients into the dry ingredients. Mix until *just* combined. Be careful not to overmix; a few lumps are okay.
Step 5: Gently fold in the fresh blueberries. If using frozen, toss them with 1 tablespoon of flour before adding.
Step 6: Transfer the batter to a piping bag (or a large Ziploc bag with a corner snipped off). Pipe the batter evenly into the prepared donut pans, filling each cavity about two-thirds full.
Step 7: Bake for 10-12 minutes, or until the donuts are golden brown and spring back when lightly touched.
Step 8: Remove from the oven and let cool in the pans for 5 minutes before carefully transferring them to a wire rack to cool completely.
Step 9: While the donuts cool, prepare the glaze: In a small bowl, whisk together the powdered sugar, milk (or lemon juice), and vanilla extract (if using) until smooth and pourable. Add more milk if needed for desired consistency.
Step 10: Once the donuts are warm but not hot, dip the top of each donut into the glaze. Let any excess drip off, then place them back on the wire rack for the glaze to set.