Preheat oven to 450F.

Place unbaked tart shells on a baking sheet.

Sprinkle coarsely chopped bittersweet chocolate onto the bottom of each shell.

In a separate bowl, whisk together the egg, packed brown sugar, light corn syrup, softened butter, vanilla extract, distilled white vinegar, and salt until well blended.

Spoon the chocolate-butter mixture into each tart shell, filling them evenly.

Bake for 12 to 14 minutes, or until the filling is puffed and bubbly, and the pastry is lightly browned.

Remove from the oven and let the tarts cool completely on the baking sheet before serving.