Preheat the oven to 350 degrees. Grease a donut pan and set aside.

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

Add the blueberries to a small bowl and sprinkle flour over them. Stir to coat, then set aside.

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

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

Gently fold the blueberries into the batter with a rubber spatula.

Add the batter to a piping bag and cut off the tip. Pipe the batter into the prepared donut pan until each well is three-quarters full.

Bake the donuts for 11-12 minutes. Let them cool for 10 minutes in the pan, then turn them out on a wire rack to cool completely. Repeat with the rest of the batter until all the donuts are baked.

To make the blueberry glaze, add powdered sugar, blueberry jam, vanilla, and milk to a large mixing bowl. Whisk until smooth, adjusting the milk for desired consistency.

Dip each donut into the glaze and return each to the wire rack. Let the icing set on the donuts for about 30 minutes before serving.