In a large bowl, combine the flour, salt, and sugar.

Add the cubed, chilled butter to the dry mixture. Using a pastry cutter or your fingers, blend until the mixture resembles coarse crumbs.

Slowly add cold water, mixing until the dough just comes together. Be careful not to overwork it.

Form the dough into a disc, wrap it in plastic wrap, and refrigerate for at least 30 minutes.

In a medium saucepan, stir together the blueberries, sugar, cornstarch, lemon juice, vanilla extract, and salt.

Cook the mixture over medium heat, stirring gently until it thickens and the blueberries start bursting, about 5-7 minutes. Remove from heat and let cool.

On a floured surface, roll out the chilled dough to about 1/8-inch thickness.

Using a sharp knife or pizza cutter, cut the dough into squares about 5 inches by 5 inches.

Place 1-2 tablespoons of the blueberry filling in the center of each square.

Fold the dough over to create a triangle shape. Press the edges with a fork to seal.

Preheat your oven to 400°F (200°C).

Place the turnovers on a baking sheet lined with parchment paper. Brush the tops with the beaten egg for a golden finish.

Bake for 20-25 minutes or until golden brown.