01 - In a large bowl, whisk together the flour, salt, and sugar. Add the cold cubed butter and, using a pastry cutter or your fingertips, cut it into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized butter pieces still visible.
02 - Drizzle in the cold water and mix gently just until the dough comes together. Be careful not to overwork it. Divide the dough into 2 equal disks, wrap each tightly in plastic wrap, and refrigerate for at least 30 minutes.
03 - Preheat the oven to 375°F (190°C). Line two baking sheets with parchment paper and set aside.
04 - On a lightly floured work surface, roll out one chilled dough disk to approximately 1/8 inch (3 mm) thick. Cut into 3 x 3 inch (7.5 cm) squares.
05 - Place a teaspoon of chopped chocolate in the center of each square. Fold two opposite corners over the filling so they slightly overlap, and gently pinch the seams to seal, forming a mini croissant shape.
06 - Arrange the shaped cookies on the prepared baking sheets. Brush the tops lightly with the beaten egg and sprinkle generously with turbinado or granulated sugar.
07 - Bake for 13 to 15 minutes, or until the tops are golden brown and the edges are crisp. Transfer to a wire rack and let cool completely before serving.
08 - Repeat the rolling, filling, shaping, and baking process with the second dough disk and remaining chocolate.