In the bowl of a food processor, pulse the flour with the butter and salt until the mixture ressembles a fine crumble. Drop in the water, and pulse again. I gave the food processor a good run until the dough wasn't moving.
Transfer the dough to your work surface and split it in 2. Give each half a good pressing to form 2 discs of dough, then wrap them tightly and put them in the fridge for 30 minutes.
Almond cream filling
Cream the butter with the icing sugar, then add the ground almonds mixing it in really well, and finally the egg and almond extract (I did all this by hand with a spatula in a medium bowl). Cover the bowl and place it in the fridge.
Assembly and finishing touches
Preheat the oven to 350 °F (175 °C) and line a cookie sheet with parchment. Roll out one half of the dough to about ⅛ inch thick, and cut out 3 ¾ inch circles (almost 4 inches).
Place 2 levelled teaspoons of almond cream in the middle of each circle, and top with 3 or 4 blueberries.
Brush the rim of each pastry round with egg and pinch into an open-centered triangle.
Refrigerate the mini pies for 30 to 45 minutes
Transfer the mini pies to the parchment-lined sheet pan. Bake the mini pies on the bottom rack for 15 minutes, then switch to the middle rack and bake for an additional 10 to 15 minutes.
Let cool then dust with icing sugar before serving.
Reroll the scraps to make more pies, and continue with the second disk of dough.