In a large mixing bowl, combine the melted butter and granulated sugar. Mix until well blended.

Add the egg yolks and vanilla extract (or vanilla bean paste) and continue mixing until the mixture is smooth.

Gently fold in the sour cream until combined.

In a separate bowl, whisk together the cake flour (or the all-purpose flour and cornstarch), baking powder, and salt.

Gradually add the dry ingredients to the wet mixture, alternating with the milk, until everything is well mixed. Finally, fold in the dried blueberries.

Lightly flour your work surface and turn out the dough. Roll it out to about 1/2 inch thickness.

Use a donut cutter or two different-sized round cookie cutters to cut out the donuts and holes.

Lay the donuts on a parchment-lined baking sheet as you work through the dough.

Heat oil in a deep skillet or fryer to 350°F (175°C).

Carefully place the donuts into the hot oil a few at a time, ensuring you don’t overcrowd the pan.

Fry them for 2-3 minutes on each side or until they’re golden brown.

Use a slotted spoon to remove the donuts and drain them on paper towels.

In a bowl, mix the powdered sugar with vanilla extract and a little milk to create a glaze.

Dunk each warm donut into the glaze, allowing excess to drip off before placing them on a cooling rack to set.