- 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.