Preheat and Prep Pan:

Heat the oven to 350°F (175°C). Butter a 6-hole donut pan lightly, then attach a large round tip to a piping bag and reserve.

Whisk Wet Ingredients:

In a mixing bowl, combine the melted butter, sugar, milk, egg, and vanilla; whisk until homogeneous.

Sift and Fold Dry:

Through a fine-mesh sieve, sift the flour into the bowl. Add the baking powder and salt, then fold with a rubber spatula until the flour is partially incorporated.

Fold in Blueberries:

Fold in the blueberries until evenly dispersed and no dry flour remains. If using frozen berries, the batter may become purple.

Pipe Batter into Pan:

Transfer the batter to the piping bag. Pipe a full ring into each donut cavity, then distribute any leftover batter evenly. Lightly smooth the tops with a moist fingertip.

Bake and Cool Donuts:

Bake for 13 to 15 minutes. Let the pan cool on a wire rack for 10 minutes, then unmold the donuts and cool fully before glazing.

Extract Blueberry Juice:

In a small microwave-safe bowl, warm 2 tablespoons of blueberries for 15 to 20 seconds until just heated. Mash with a fork and press through a fine sieve placed over the same bowl to collect the juice; discard solids.

Make and Apply Glaze:

Sift the powdered sugar into a bowl. Add the vanilla and 1/2 teaspoon of blueberry juice; stir thoroughly. Continue adding blueberry juice 1/2 teaspoon at a time until the glaze is thick and flows in ribbons. Drizzle over the cooled donuts and let set for 20 minutes before serving.