Preheat your oven to 350°F (175°C) and lightly grease your donut molds.

In a large mixing bowl, combine the melted butter, milk, baking powder, baking soda, sugar, and vanilla extract. Stir until smooth.

Gradually add the all-purpose flour to the wet mixture, stirring until just combined. Avoid overmixing.

Gently fold in the blueberries with a spatula, ensuring they're evenly distributed.

Pour the batter into the donut molds, filling each cavity about two-thirds full.

Bake for approximately 20 minutes, until golden brown and a toothpick comes out with moist crumbs.

Let the donuts cool in the pan for about 5 minutes, then transfer to a wire rack. Prepare the glaze if desired.