Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 12-cavity donut pan (or two 6-cavity pans) with non-stick spray or butter.
Step 2: In a large bowl, whisk together the flour, granulated sugar, baking powder, baking soda, salt, and cinnamon (if using). Make sure there are no lumps.
Step 3: In a separate medium bowl, whisk together the egg, buttermilk, melted butter, and vanilla extract until well combined.
Step 4: Pour the wet ingredients into the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. Be careful not to overmix; a few lumps are okay.
Step 5: Gently fold in the fresh blueberries. If using frozen blueberries, do not thaw them first; toss them directly into the batter.
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 cavities, filling each about two-thirds full.
Step 7: Bake for 12-15 minutes, or until the donuts are golden brown and spring back when lightly touched. A toothpick inserted into a donut should come out clean.
Step 8: Remove the donut pan from the oven and let the donuts cool in the pan 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 medium bowl, whisk together the powdered sugar, milk (or lemon juice), and vanilla extract until smooth. Adjust the consistency by adding more sugar for a thicker glaze or more milk for a thinner glaze.
Step 10: Once the donuts are cool enough to handle but still slightly warm, dip the top of each donut into the glaze, letting any excess drip off. Place them back on the wire rack for the glaze to set. Serve your delicious **Baked Blueberry Donuts** immediately or store for later.