Preheat your oven to 350°F. Grease a donut pan and set it aside.

In a small bowl, whisk together the flour, baking powder, salt, and cardamom (if using). Set aside.

In another small bowl, toss the blueberries with 1 tablespoon of flour to coat them. This helps prevent them from sinking to the bottom while baking.

In a large bowl, whisk together the melted butter, sugar, eggs, vanilla, sour cream, and milk.

Add the dry ingredients to the wet ingredients and mix until just combined.

Gently fold in the blueberries with a rubber spatula.

Transfer the batter to a piping bag and cut off the tip. Pipe the batter into the prepared donut pan, filling each well about three-quarters full.

Bake for 11-12 minutes. Let the donuts cool in the pan for 10 minutes, then transfer them to a wire rack to cool completely. Repeat with the remaining batter.

In a large mixing bowl, combine the powdered sugar, blueberry jam, vanilla, and milk. Whisk until smooth. Start with 2 tablespoons of milk and add more if needed to reach the desired consistency.

Dip each cooled donut into the glaze and place back on the wire rack. Let the glaze set for about 30 minutes before serving.