Preheat your oven to 350°F (175°C) and grease a donut pan with non-stick spray.
In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, salt, and cinnamon until well combined.
In another bowl, mix the melted butter, eggs, buttermilk, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir until just combined. Be careful not to overmix.
Gently fold in the blueberries, ensuring they are evenly distributed throughout the batter.
Using a piping bag or a spoon, fill each donut cavity about 2/3 full with the batter.
Bake in the preheated oven for 10-12 minutes, or until a toothpick inserted into the center comes out clean.
Allow the donuts to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.