Preheat the oven to 375°F and line 2 baking sheets with parchment paper. Set aside.

Slice the stale croissants in half lengthwise and place on the baking sheets with the cut sides up.

In a microwave safe bowl, combine the water and sugar and microwave for 1 minute, stirring until the sugar crystals have completely dissolved. If they haven’t dissolved, microwave for an additional 30 seconds. Stir in the almond extract and set aside.

In a medium bowl, combine the butter, egg, almond meal, granulated sugar, almond extract, vanilla extract, and salt. Beat together until smooth.

Use a pastry brush to generously coat the cut sides of the croissants.

Spread about 1 tablespoon of the almond filling on the bottom halves of each croissant and place the top halves back on to sandwich them together.

Brush the tops of the croissants with simple syrup and top each with another heaping tablespoon of the almond filling, spreading it over the top.

Top each croissant with sliced almonds, pressing the almonds into the topping to ensure they stick.

Bake for 12 to 15 minutes in the preheated oven or until golden brown.