Sift flour and salt in a medium size bowl.

Cream butter and sugar in the bowl of an electric mixer until light and fluffy. Beat in egg. Reduce the speed to low and beat in the flour mixture until incorporated.

Gather dough and form it into a disk. Wrap the disk with plastic wrap and refrigerate for 20 minutes.

Place 4 tartlet pans on a cookie sheet.

On a lightly floured surface, roll out the dough to 1/4 inch thickness. Cut into 4 6-inch rounds. Re-roll the scraps as needed. Line the bottom and sides of the prepared pans with the dough circles. Use a small kitchen knife to trim off the excess dough. Prick the bottom of each tartlet with a fork a few of times.

Place in the refrigerator for about 15 minutes while oven is preheating to 350 degrees F.

Remove tartlets from refrigerator and put them into the oven. Bake for 20 to 25 minutes or until lightly golden. Remove from the oven and let the tart shells to cool completely.

Add cream and sugar to a medium saucepan. Bring to a boil over medium heat.

Once boiling, remove from heat and stir in butter, dark chocolate, espresso powder, and salt. Finally stir in milk.

Pour into cooled tart shells. Refrigerate for at least 2 hours, or until set.

Before serving, carefully remove tarts from pans and sprinkle with flaked sea salt.