Preheat your oven to 350°F (175°C) and grease your donut pan.
In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, salt, and cinnamon.
In another bowl, combine the melted butter, buttermilk, eggs, and vanilla extract. Mix well.
Pour the wet ingredients into the dry ingredients and stir gently until just combined.
Gently fold in the blueberries into the batter.
Fill each cavity of the donut pan about 2/3 full with the batter.
Bake for 12-15 minutes or until a toothpick comes out clean.
Allow the donuts to cool in the pan for about 5 minutes before transferring to a wire rack.