Preheat your oven to 350°F. Line a baking sheet with parchment paper.
In a small saucepan over medium heat, combine 3/4 cup water and 1/3 cup granulated sugar. Stir until the sugar dissolves, about 2 minutes. Remove from heat and let cool slightly.
3/4 cup water, 1/3 cup granulated sugar
In a bowl, beat 9 tbsp unsalted butter softened and 1/2 cup granulated sugar together until light and fluffy. Add 3 eggs one at a time, mixing well after each. Stir in 1 cup almond flour, 1 tsp almond extract, 1/2 tsp vanilla extract, and 1/2 tsp salt until smooth and combined. The frangipane should be thick and spreadable.
9 tbsp unsalted butter, 1/2 cup granulated sugar, 1 cup almond flour, 3 eggs, 1 tsp almond extract, 1/2 tsp vanilla extract, 1/2 tsp salt
Slice each of the 6 croissants in half horizontally like a sandwich bun. Brush the cut sides of each half generously with the simple syrup, don't be shy, you want the croissant to absorb it fully for that soft, bakery-style interior.
6 croissants
Spread a generous layer of frangipane (about 2 tablespoons) on the bottom half of each croissant. Place the top half back on, then spread another thin layer of frangipane over the top of each croissant.
Press a generous pinch of 1/2 cup sliced almonds onto the frangipane-topped surface of each croissant so they stick. Place on the prepared baking sheet.
1/2 cup sliced almonds
Bake for 15–18 minutes, until the frangipane is set, the almonds are golden, and the croissants are deep amber and crispy on the outside.
Let cool for at least 5 minutes, then dust generously with 3 tbsp powdered sugar, for dusting. Serve warm or at room temperature.
3 tbsp powdered sugar