1. Preheat the oven to 375°F/ 190°C. Line a baking sheet with parchment paper or a silicone baking mat.

Frangipane

1. With a hand or stand mixer fitted with the paddle attachment, beat together the butter and sugar until it's light and fluffy.
2. Beat in the egg and vanilla until well combined.
3. Mix in the almond flour, all purpose flour, and salt just until combined.

Simple Syrup

1. Add the water and sugar to a small microwave safe bowl. Microwave in 30 second intervals and stir after each interval until the sugar has dissolved.

Assembly

1. Slice open each croissant lengthwise, leaving it slightly attached at the end.
2. Brush the simple syrup on the open portion of each croissant.
3. Transfer the frangipane to a piping bag fitted with a large round tip (for easy distribution, optional). Pipe a generous amount of frangipane on to one side of each croissant, and smooth it out with a knife. Be sure to reserve a small amount of frangipane for the top of each croissant.
4. Fold the top half back on top of the croissants, then pipe a line of frangipane on top of each croissant. Use a knife to slightly spread out the frangipane on top.
5. Press the sliced almonds onto the frangipane on top of each croissant.
6. Bake the croissants for 10-15 minutes or until the frangipane starts to turn golden brown.
7. Dust with powdered sugar, serve, and enjoy!