Defrost puff pastry and roll out. Cut into rectangles measuring 6 by 8 cm, I got 12 rectangles. On 6 rectangles, make 3 longitudinal cuts.
Put 1-2 tsp on whole rectangles. blueberries, sprinkle with 1 tsp. brown sugar. Top with cut rectangles. Press the edges of the puffs tightly.
Place on a greased baking sheet, brush with egg yolk and sprinkle with brown sugar. Bake in an oven preheated to 200 degrees for 20-25 minutes.