Step 1: Preheat your oven to 350°F (175°C). Lightly grease a donut baking pan with cooking spray or butter to prevent sticking.
Step 2: In a large mixing bowl, combine the all-purpose flour, granulated sugar, baking powder, baking soda, salt, and ground cinnamon. Whisk these dry ingredients together thoroughly.
Step 3: In a separate medium bowl, whisk together the melted unsalted butter, large eggs, buttermilk (or milk with vinegar substitute), and vanilla extract until the mixture is smooth and well combined.
Step 4: Pour the wet ingredient mixture into the bowl of dry ingredients. Stir gently with a spoon or spatula until just combined. Be careful not to overmix the batter. Gently fold in the fresh or frozen blueberries.
Step 5: Transfer the donut batter into the prepared donut pan. For easier filling and a neater finish, you can use a piping bag or a Ziploc bag with a corner snipped off to pipe the batter into each cavity, filling them about ¾ full.
Step 6: Bake the donuts in the preheated oven for 12-15 minutes, or until the edges are golden brown and a toothpick inserted into the center of a donut comes out clean.
Step 7: Remove the donut pan from the oven and let the donuts cool in the pan for about 5 minutes. Carefully transfer the warm donuts to a wire rack to cool completely. While they cool, prepare the glaze (if desired).
Step 8: To make the glaze, whisk together the powdered sugar, milk, and vanilla extract in a small bowl until smooth. Once the donuts are completely cooled, drizzle the glaze over them. Serve immediately and enjoy!