Dip mint leaves in beaten egg white and coat with sugar.
Place leaves on a wire rack to dry for several hours or overnight.
Sieve flour and icing sugar into a bowl with salt and lemon zest.
Whisk egg with sour cream and 2 tablespoons of water, then pour into the flour.
Knead the mixture into a smooth dough on a work surface.
Cover dough with clingfilm and rest for 1 hour.
Roll dough to 1-2mm thickness and cut into 8cm rounds.
Toss blueberries in caster sugar.
Place 4-5 blueberries on each round, fold into crescents and seal edges.
Boil pierogi in a large pot for 2-3 minutes until they float.
Fry pierogi in butter with sugar and cinnamon until browned.
Whisk icing sugar into sour cream.
Serve dumplings with sweetened cream, mint leaves, and icing sugar.