Preheat the oven to 375F (190C) and line a baking sheet with parchment paper.
Pour the water into a medium-sized saucepan, add the sugar and almond extract, and bring to a boil over medium heat.
Reduce heat to a simmer, and cook for 3-4 minutes. Stir as you cook.
Remove from the heat, and pour the syrup into a wide bowl.
Slice the cornetti (or croissants) horizontally, then dip each half face down into the simple syrup for 1-2 seconds.
Place the bottom halves over the prepared baking sheet, then spread, or pipe 1-4 tablespoons of the almond cream.
Top with the top halves, then top each pastry with 1-2 tablespoons of almond cream.
Sprinkle with the sliced almonds and bake for 15-18 minutes if filled with 1-2 tablespoons. Or, 25-28 minutes if filled 3-4 tablespoons. The pastries are ready when the sliced almonds are golden brown, and the filling is no longer raw (lift the top part if you are unsure).If the tops are dark but the pastries are not ready, top them with a piece of aluminum foil.
Notes
You have the flexibility to fill the pastries with anywhere from one tablespoon to as much as four or even five tablespoons of almond cream. Keep in mind that the baking time will vary accordingly – the more cream you use, the longer the baking time. Refer to the recipe card for specific details.
Instead of soaking, use a pastry brush to coat the pastries with almond syrup.
Cornetti typically gets a richer golden brown color compared to the classic croissant. Cover the pastries with aluminum foil if you notice that the almonds and the top of the cornetti (or croissant) are baking too quickly and may risk burning.
Incorporate 2-3 tablespoons of Amaretti (almond liquor) into the syrup for a more pronounced almond flavor.
This recipe produces 5-8 large Cornetti or croissants or 12-15 small pastries. Double the almond paste recipe and the sweet, simple syrup to increase the quantity.
Allow the pastries to cool in the pan for 20 minutes before transferring them to a wire rack.
Homemade cornetti don't require staleness if they are at room temperature and the syrup is warm. If you only have fresh, not day-old croissants, slice and bake them for about 10 minutes to achieve the desired dryness."