Egg whites should be at room temperature, and the butter should be cold. 1. Preheat the oven to 410°F (210°C). 2. Cut the butter into small pieces. 3. In a large bowl, mix the flour and baking powder with a whisk. 4. In the mixer bowl with the wire whip attachment, add the butter, sugar, and salt; beat until the butter turns white. 5. With the mixer on low speed, add the egg and yolks one at a time. Use a spatula to occasionally scrape the cream down the sides of the bowl. 6. Change the mixer attachment to the flat beater, add the flour mixture, and knead the dough. Wrap the dough in plastic wrap and refrigerate for 30 minutes. 7. Fill the molds with the dough. Work quickly, pressing the dough firmly into the bottom and sides of the molds, leaving a hollow in the middle to form tartlets. Place the molds on a baking sheet and bake for 10–15 minutes. 8. Let the tartlets cool for 5 minutes. Remove from the molds onto a wire rack and fill the molds with jam, preferably using a pastry bag. 9. CREAM: Using the wire whip attachment in a clean and dry mixer bowl, mix the egg whites with a pinch of salt on low speed until foamy. Add vanilla and increase the mixer speed to high. While the whites are whipping, pour the sugar into a saucepan, add water, and cook until it reaches the soft-ball stage (244°F / 118°C). 10. Add vinegar to the whites, reduce the mixer speed to low, and, without stopping the mixer, pour the hot syrup in a thin stream into the egg whites. Once all the syrup has been added, increase the speed to high and beat for 30 seconds. The cream is ready. 11. Place the cream in a pastry bag and fill the tartlets. 12. Preheat the oven to 410°F (210°C) again. Return the tartlets to the baking sheet and place them in the oven for 2 minutes to form a thin crust on top. Remove from the oven and sprinkle with powdered sugar if desired.