If you don't have a kitchen torch, you can use your broiler. Chill the tarts in the freezer for twenty minutes, and then place them on a baking sheet. Broil until tops are golden brown, two to three minutes. For the best results, prepare these tarts no more than one day in advance, and wait to caramelize them until just before serving. This way, the tart shells will remain firm and crumbly, and the sugary topping will keep its crunch.