Preheat your oven to 350°F (175°C). Line a large baking sheet with parchment paper or a silicone baking mat.

Make the almond cream by beating the room-temperature butter and granulated sugar together in a medium bowl until light and fluffy, about 2 minutes. Add the almond flour, one egg, almond extract, vanilla extract, and salt, then beat until smooth and fully combined.

Using a serrated knife, carefully slice each croissant horizontally without cutting all the way through, leaving one long edge intact as a hinge. Gently open each croissant like a book.

Spoon or pipe about 2-3 tablespoons of almond cream into the center of each opened croissant, spreading it evenly across the bottom half. Close the croissants gently to encase the filling.

Beat the remaining egg in a small bowl and lightly brush the top of each filled croissant with the egg wash. Spread a thin layer of the remaining almond cream on top of each croissant, then sprinkle generously with sliced almonds.

Arrange the croissants on the prepared baking sheet with at least 2 inches of space between them. Bake for 18-22 minutes, until the almond topping is deep golden brown and the croissant layers look crisp and flaky.

Remove from the oven and let the croissants cool on the baking sheet for 5 minutes to allow the filling to set slightly. Transfer to a wire rack and dust with powdered sugar just before serving warm.