Preheat your oven to 350°F (175°C). Generously grease a 6-cavity donut pan with non-stick spray or melted butter.

In a medium-sized bowl, whisk together the cake flour, granulated sugar, baking powder, salt, and nutmeg until well combined.

In a separate smaller bowl, whisk the buttermilk, egg, melted butter, and 1 teaspoon of vanilla extract until smooth.

Pour the wet ingredients into the bowl with the dry ingredients. Using a spatula, gently fold the mixture together until it is just combined. A few small streaks of flour should still be visible.

Gently fold in the fresh blueberries using as few strokes as possible to avoid crushing them. The batter will be thick.

Transfer the batter to a piping bag or a large zip-top bag with the corner snipped off. Pipe the batter evenly into each donut cavity, filling them about two-thirds full.

Bake for 12-15 minutes, or until a wooden skewer inserted into the center of a donut comes out clean. The tops should spring back lightly when touched.

Let the donuts cool in the pan for 5 minutes before carefully transferring them to a wire rack to cool completely.

While the donuts cool, prepare the glaze. In a small, shallow bowl, whisk together the powdered sugar, 1/2 teaspoon of vanilla extract, and 2 tablespoons of milk until smooth.

If the glaze is too thick, add more milk, a teaspoon at a time, until it reaches a thin but not watery consistency.

Once the donuts are completely cool, dip the top of each donut into the glaze, allowing any excess to drip back into the bowl. Place the glazed donuts back on the wire rack to let the glaze set for about 10-15 minutes before enjoying.