For the dough: The day before, peel potatoes, rise and boil. Press potatoes through a ricer and knead with flour, cornstarch, egg yolks and salt. Let cool for 30 minutes.
For the blueberries: Rinse blueberries. Beat eggs with sugar until fluffy. Stir in sour cream. Grease 16 muffin wells with butter and dust with breadcrumbs. Roll out dough on lightly floured surface and divide into 16 portions. Press each portion into a well and create and edge. Sprinkle dough with breadcrumbs. Add blueberries to well and top with cream. Bake in an oven preheated to 200°C (approximately 400°F) for 20-25 minutes.
Remove and serve dusted with powdered sugar.