Lightly coat donut pan(s) with cooking spray and flour.
In a large bowl, whisk together dry ingredients.
In separate bowl, stir wet ingredients until well-combined. Lightly coat blueberries in flour.
Add wet ingredients to dry, mixing just until combined. (Go gently as over-mixing can lead to dry donuts.) Fold in blueberries.
Fill each donut pan well ¾ full without covering middle section (so you're left with a hole after baking). Be sure to evenly distribute blueberries.
Bake for 12-14 minutes, or until edges are golden brown and top springs back when lightly touched. Allow to cool for 5 minutes in pan, then transfer donuts upside down onto cooling rack. Completely cool before glazing.
Instructions to Make Glaze
In a small bowl, mix powdered sugar with lemon juice. Slowly add milk (a bit at a time) and stir until smooth. Add more powdered sugar if glaze becomes too runny.
Dip bottom of donuts into glaze, then place glazed-side up onto rack to drip dry. Place parchment paper or baking sheet below to catch drippings. Allow to set up, then transfer to airtight container.
Notes
For the gluten-free all-purpose flour, I put together my own using this recipe (with xanthan gum). I have not tested this recipe with regular all-purpose flour, but will update when I do.
To bring eggs to room temperature, place in a bowl with warm water. Allow to sit for about 5 minutes.
Can I use frozen blueberries? You can, but don't thaw. I suggest using fresh. You may need to adjust baking time a bit longer.
The lemon glaze for donuts shouldn't be too thin or it will seep in. Add a bit more powdered sugar if it's runny.
How long will these donuts keep? Store glazed donuts in an airtight container at room temperature for up to 3 days. These donuts are best eaten within a day or two. We like them slightly warmed up.
Donuts can be frozen up to 3 months in an airtight container or freezer bag. Allow the glaze to fully set up before popping into the freezer. For best results, glaze after thawing (for at least one hour).