Preheat the air fryer to 270°F (132°C).
Stir together the almond flour, powdered sugar, water, egg, almond extract, and salt until you have a smooth, spreadable paste. Set aside.
Stir the granulated sugar, hot water, and vanilla bean paste together until the sugar fully dissolves.
Slice the croissants horizontally and brush the cut sides generously with the vanilla syrup.
Spread about 2 tablespoons of the almond filling over the bottom half of each croissant. Cook's tip: dip your offset spatula in water to spread the filling more easily.
Close the croissants and brush the tops with more syrup.
Spread about 2 tablespoons of the remaining filling over the tops and sprinkle with sliced almonds, pressing them lightly into the paste so they stay put.
Place the croissants in the preheated basket in a single layer, not touching. Cook at 270°F (132°C) for 15 to 18 minutes, until deep golden brown.
Dust with powdered sugar as soon as they come out of the air fryer.
Let cool 10 minutes before serving — the filling will be very hot.