01 - In a large mixing bowl, whisk together the flour, salt, and sugar. Add the cold cubed butter. 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 still visible.
02 - Drizzle in the cold water and mix gently until the dough just 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. Line two baking sheets with parchment paper and set aside.
04 - On a lightly floured surface, roll out one chilled dough disk to approximately 1/8-inch thickness. Cut into 3×3-inch squares using a knife or pastry wheel.
05 - Place a teaspoon of chopped chocolate in the center of each dough square. Fold two opposite corners over the filling so they slightly overlap, and gently pinch the dough together to seal, forming a mini croissant shape.
06 - Arrange the shaped cookies on the prepared baking sheets, spacing them about 1 inch apart. Brush the tops with the beaten egg and sprinkle generously with turbinado or granulated sugar.
07 - Bake for 13 to 15 minutes, rotating the pans halfway through, until the cookies are golden brown and crisp on the edges. 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 the remaining chocolate filling.