1Preheat oven to 450 degrees F (230 degrees C). Place tart shells on a baking sheet.
2Chop 3 ounces of the chocolate into small pieces; sprinkle evenly into the bottom of each tart shell.
3Whisk brown sugar, corn syrup, egg, butter, vinegar, vanilla, and salt together in a bowl; fill tart shells with sugar mixture 3/4 full.
4Bake in the preheated oven until filling expands and is bubbly and pastry is lightly golden, about 12 minutes. Remove from the oven to cool.
5Melt the remaining chocolate in a microwave-safe glass or ceramic bowl in 30-second intervals, stirring after each melting, about 1 minute (depending on your microwave). Drizzle chocolate over cooled tarts.