Make the Simple Syrup (optional): In a small saucepan, combine the water and sugar. Heat over medium heat, stirring until the sugar is completely dissolved. Remove from heat and let it cool.

Prepare the Frangipane: In a mixing bowl, cream together the softened butter and sugar until light and fluffy.

Add the Egg and Extracts: Beat in the egg until well combined. Then, stir in the almond and vanilla extracts.

Combine Dry Ingredients: In a separate bowl, whisk together the almond flour, all-purpose flour, and salt.

Combine Wet and Dry: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Don't overmix.

Prepare the Croissants: Slice each croissant horizontally, almost all the way through, like you're making a sandwich.

Assemble the Croissants: Brush the insides of each croissant with the simple syrup (if using). Then, spread a generous layer of frangipane on the bottom half of each croissant.

Add Top Layer: Close the croissants, and spread a thin layer of frangipane on top of each one.

Add Almonds and bake: Sprinkle the sliced almonds evenly over the frangipane on top of the croissants.

Bake: Place the croissants on a baking sheet lined with parchment paper. Bake in a preheated oven at 350°F (175°C) for 15-20 minutes, or until the frangipane is golden brown and the almonds are toasted.

Cool and Dust: Let the croissants cool slightly on the baking sheet before transferring them to a wire rack. Once cooled a bit, dust generously with powdered sugar.