Place the puff pastry sheets side by side on a baking sheet lined with baking paper and spread half of the butter on top.
Wash, peel, quarter and core the apples. Cut the fruit pieces into fine slices and spread them on the puff pastry sheets. Drizzle the apples with lemon juice and then sprinkle with sugar.
Bake the tartlets on the middle rack for about 15 minutes. In the meantime, melt the remaining butter, brush the parts with it and bake for another 10 minutes.
Take the tartlets out of the oven and let them cool. Sprinkle with powdered sugar. They taste best when they are lukewarm. Suffice it with some whipped cream!