Yes, you can definitely prepare Hazelnut Tartelettes in advance, making them an excellent choice for gatherings or special occasions. After baking, allow the tartlets to cool completely before storing them. They can be kept in an airtight container at room temperature for up to 2 days, though for the best taste and texture, it's advisable to refrigerate them where they can last for up to a week. If you prefer, you can also freeze them: wrap each tartlet individually in plastic wrap and then place them in a freezer-safe bag. They’ll keep well for up to 3 months. When ready to enjoy, simply thaw them in the refrigerator overnight and, if you like, heat them briefly in the oven for that fresh-baked touch.