In saucepan, stir together blueberries, sugar and flour; add lemon juice and 1/4 cup/60 mL water. Simmer over low heat until blueberries are soft and sauce is thickened, about 5 minutes. Set aside.

Dough: In large bowl, whisk together flour and baking powder. In separate bowl, whisk together egg, milk and butter; stir into flour mixture. Add cold water, 1 tbsp/15 mL at a time (6 to 7 tbsp/90 to 100 mL total), until soft dough is formed. Knead until dough is smooth. Cover with plastic and let rest for 10 minutes.

Filling: Whisk together sugar and flour; set aside.

Roll dough to scant 1/4-inch/5 mm thickness. Cut out 3-inch/8 cm rounds. Stretch out dough slightly and fill each round with scant 1 tsp/5 mL flour mixture and 1 tbsp/15 mL blueberries. Pull dough over filling; pinch edges together to seal. Continue until dough and filling are used up, letting reworked scraps rest slightly before rolling (keep unrolled dough and filled dumplings covered with a clean tea towel).

In large pot of lightly salted water, boil dumplings, in batches if necessary, until dough is tender at thickest edges, about 10 minutes. Remove with slotted spoon to serving plate; drizzle with butter to prevent sticking. Serve with blueberry sauce and sour cream on the side.

Makes 32 to 36 dumplings.