Preheat your oven to 350°F and grease a donut pan.
In a large bowl, whisk together the flour, sugar, baking powder, salt, and cinnamon.
In another bowl, combine the melted butter, eggs, buttermilk, and vanilla extract. Mix until well combined.
Pour the wet ingredients into the dry ingredients and stir until just combined. Gently fold in the blueberries.
Spoon the batter into the prepared donut pan, filling each cavity about 3/4 full.
Bake for 12-15 minutes or until a toothpick inserted into the center comes out clean. Let the donuts cool in the pan for 5 minutes before transferring them to a wire rack.
While the donuts cool, prepare the glaze by whisking together the powdered sugar, milk, and vanilla extract until smooth.
Dip the cooled donuts into the glaze and place them back on the wire rack to set.