### Instructions:
1. **Preheat and Prepare:** Preheat your oven to 350°F (175°C). Lightly grease a 6-cavity donut pan.
2. **Combine Dry Ingredients:** In a large bowl, whisk together the flour, granulated sugar, baking powder, and salt.
3. **Combine Wet Ingredients:** In a separate medium bowl, whisk together the milk, melted butter, egg, vanilla extract, and lemon zest.
4. **Combine Wet and Dry:** Pour the wet ingredients into the dry ingredients and mix until just combined. Be careful not to overmix; a few lumps are okay.
5. **Fold in Blueberries:** Gently fold in the blueberries. If using frozen blueberries, toss them with a tablespoon of flour before adding to prevent them from sinking.
6. **Fill Donut Pan:** Carefully spoon the batter into the prepared donut pan, filling each cavity about two-thirds full.
7. **Bake:** Bake for 12-15 minutes, or until the donuts are golden brown and a toothpick inserted into the center comes out clean.
8. **Cool Donuts:** Let the donuts cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
9. **Make the Glaze:** While the donuts are cooling, prepare the lemon glaze. In a small bowl, whisk together the powdered sugar and lemon juice until smooth. If desired, stir in the additional lemon zest.
10. **Glaze the Donuts:** Once the donuts are completely cool, dip the top of each donut into the lemon glaze, allowing any excess to drip off. Place them back on the wire rack for the glaze to set.
### Short Tips:
* **Don’t overmix the batter!** Overmixing can lead to tough donuts.
* **For extra tang:** Add a tiny pinch of citric acid to the glaze.
* **No donut pan?** You can make these as muffins or even bake them in a mini Bundt pan. Adjust baking time accordingly.
* **Storing:** Store leftover donuts in an airtight container at room temperature for up to 2-3 days.
