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