Preheat the oven to 375°F (190°C) and line a rimmed baking sheet with parchment paper.

In the bowl of a stand mixer, combine almond flour, sugar, unsalted butter, and kosher salt. Beat on medium speed until well combined, about 2 minutes.

Add the egg and almond extract, then beat on medium to medium-high speed until light and fluffy, about 2 minutes more.

In a medium saucepan, combine water and granulated sugar. Set over medium-high heat and bring to a boil, stirring occasionally. Once the sugar is dissolved, remove from heat and stir in amaretto. Cool slightly.

Using a serrated knife, slice each croissant in half horizontally.

Dip the bottom half of each croissant into the simple syrup, submerging it fully. Allow excess syrup to drip off, then transfer to the baking sheet. Repeat with all 4 halves. Do not dip the top croissant.

Spread 2 tablespoons of frangipane filling on each bottom half of the croissants. Place the top half over the filling and press firmly to flatten.

Brush with a bit of the remaining simple syrup, then evenly spread the remaining frangipane on top. Finish with a generous sprinkle of sliced almonds.

Transfer to the oven and bake for 15-18 minutes. Allow to cool for 10 minutes, then dust with confectioners' sugar.