Combine the flour, sugar and salt in a bowl.
Divide the butter into small cubes and add it to the flour; crumble together or use a food processor until it starts to hold together.
Add cold water a little at a time, and quickly mix together.
Divide the dough into two pieces and cover with cling film. Leave to rest for about 1 hour.
Meanwhile, boil the blueberries with the sugar and flavourings for about 5 minutes.
Blend using a hand blender. Leave to cool.
Pre-heat the oven to 200 °C.
Take the dough out of the fridge and knead it until smooth.
On a floured surface, roll out the pieces of dough into two thin pieces of equal size.
Spread a thin layer of the blueberry purée on one half.
Slice the other piece into thin strips, and lay it over the blueberries in a criss-cross pattern.
Cut into rectangular pieces or cut out small circles, about 3 cm in diameter, and place on a parchment-lined baking sheet.
Glaze the cookies with egg and sprinkle with cane sugar.
Bake in the oven for 10-14 minutes.
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