Preheat the oven to 355°F (180°C). Line a baking sheet with a silicone mat or parchment paper.

To make simple syrup, pour water with sugar into a small saucepan and bring to a boil. Once the sugar is completely dissolved, remove it from the heat. Optionally, add vanilla extract or rum and let it cool.

To make almond croissant filling, mix the softened butter and icing sugar with a hand whisk or a hand mixer for 2-3 minutes until the mixture is smooth and creamy.

Add the egg and mix well with a whisk. The mixture may look a little bit curdled, but it will disappear with the addition of ground almonds.

Using a wooden spoon or rubber spatula, fold in ground almonds and mix again until you obtain a smooth cream.

To assemble croissants, use a large serrated knife to cut the croissants in half lengthwise. With a silicone or pastry brush, brush each half of the croissant with lukewarm sugar syrup, and let them soak for 1-2 minutes on a wire rack.

Spread about 2 tablespoons of almond cream on the bottom half of each croissant. Place the top half of the croissant and press lightly with your hand to seal. Spread about 1 tablespoon of almond cream on top of each croissant. Take one croissant at a time and wrap the cream-covered top into the bowl of flaked almonds.

Continue for the rest of the croissants. Place croissants on the prepared baking sheet and sprinkle with fleur de sel (optionally).

Bake for 13-15 minutes. The almond cream should be set, and the tops of the croissants should be lightly browned. Remove from the oven and let cool on a cooling rack. Sprinkle croissants with icing sugar.