In a large mixing bowl, combine yeast and warm water. Let this mixture stand for about 5 minutes until the mixture is frothy. Add the warm milk, butter, sugar, eggs, cardamom and salt. Stir everything well with a wooden spoon.
Add 3 cups of flour, one cup at a time, stirring well after each addition. Add just enough of the remaining flour, a few tablespoons at a time, just until the dough gathers in the center of the bowl as you stir. You may not need the entire amount of flour.
Turn the dough on a lightly floured board. Knead the dough until it is smooth, elastic and has lost some stickiness. Shape it into a ball and place it inside a bowl. Cover it with a clean kitchen towel and let it rise for 1 to 1 and 1/2 hours, or until the size is doubled. Meanwhile, prepare the blueberry filling. Combine the sugar, cornstarch and blueberries in a bowl and stir well.
Once risen, gently deflate the dough and divide it into 8 equal portions. Shape each portion into a smooth ball. Arrange the dough balls in a large baking pan that is lined with parchment paper. Cover it once again with a kitchen towel and let it rise for about 40 minutes. The dough should be puffy, soft and has increased n size.
Preheat oven to 350 F. Use your hands to make a large well in the center of a dough ball. Press it down firmly until your hand touches the surface underneath. Spoon as many blueberries you can fit in the well, brushing off the excess powders in the berries. Repeat with the remaining balls, leaving out the excess cornstarch mixture in the bowl. Brush the edges with a beaten egg.
Bake the buns at 350 F for 18 -23 minutes, or until the buns are lightly golden. Serve warm. Reheat leftovers in the microwave for 8-12 seconds before serving.