In a large bowl, whisk together the flour and salt. Add the cold butter pieces and use a pastry blender or your fingers to cut the butter into the flour until the mixture resembles coarse crumbs. Slowly add the ice water, one tablespoon at a time, mixing until the dough just comes together. Be careful not to overmix.
Form the dough into a disc, wrap it in plastic wrap, and refrigerate for at least 30 minutes. This chilling time is crucial for preventing the gluten in the flour from developing too much.
On a lightly floured surface, roll out the dough to about 1/8-inch thickness. Use a cookie cutter or a knife to cut out circles slightly larger than the mini tart pans. I use a 2.5-inch cookie cutter for mine.
Gently press the dough circles into the mini tart pans, making sure they fit snugly against the sides. Trim any excess dough from the edges. You can use a fork to prick the bottom of each tart shell to prevent them from puffing up during baking.
Preheat your oven to 375°F (190°C). Line the tart shells with parchment paper and fill with pie weights or dried beans to prevent them from shrinking. Bake for 15 minutes, then remove the parchment paper and pie weights and bake for another 5-7 minutes, or until the crusts are lightly golden brown. Let them cool completely.
While the crusts are cooling, prepare the chocolate filling. In a heatproof bowl set over a saucepan of simmering water (double boiler), combine the chopped chocolate, heavy cream, and butter. Stir until the chocolate and butter are melted and the mixture is smooth. Remove from heat and stir in the salt.
Pour the chocolate filling into the cooled tart shells, filling them almost to the top.
Refrigerate the tarts for at least 1 hour, or until the filling is firm.
Just before serving, sprinkle the Mini Chocolate Tarts with Sea Salt flakes. This adds a wonderful flavor contrast and a touch of elegance.