Preheat oven to 350° Fahrenheit (176° Celsius). Spray mini donut pan with cooking spray. Lightly wipe out any excess oil. Set aside.
In a large bowl, whisk together dry ingredients (flour, brown sugar, baking powder, and salt).
In a separate bowl, combine melted butter with vanilla extract, milk, and Greek yogurt. Add the lightly-beaten egg and whisk until combined.
Pour the wet ingredients into the dry ingredients. Stir gently with a spatula until just a few flour streaks remain. Gently fold in blueberries until no flour streaks remain. (Don't over-mix.)
Spoon the batter into a piping bag or a Ziploc bag with the corner cut off. (The hole should be large enough for the blueberries to pass through). Pipe the batter into each donut pan cavity, only filling them 3/4 full.
Bake for 8-10 minutes, until lightly golden. A toothpick inserted in the center should come out clean. Remove from the oven. Once the donuts are cool enough to handle, carefully pry them from the pan with a fork and transfer them to a wire rack to cool.
Make the blueberry glaze:
Cook the blueberries in a saucepan on medium low heat, stirring continuously. As the blueberries soften, mash them with a wooden spoon to release the juices.
Place a sieve/strainer over a small bowl. Pour the blueberry mixture into the strainer. Press the wooden spoon over the blueberries to release as much of the the juice as possible into the bowl. (Discard the blueberry skins, or use them in another recipe.)
Add powdered sugar, vanilla extract, and lemon juice to the blueberry puree. Stir until combined. The consistency should be thick (it should drip slowly if drizzled from a spoon.) Add more powdered sugar if needed.
Glaze & top the donuts:
Place a piece of parchment or wax paper below a wire cooling rack. Dip each donut in the glaze about halfway, allowing some excess to drip off. Place the glazed donuts on the wire rack until the glaze hardens. Then, enjoy!
Notes
*If you aren't a lemon fan, replace the lemon juice with an equal amount of milk in the blueberry glaze.Storage Tips: These donuts are best enjoyed the same day they're made, but can be stored on the counter in an airtight container for up to 2 days. The glaze should only be added the same day you're planning to eat the donuts. (Otherwise, they can get soggy!)