In a small saucepan, combine 1/2 cup granulated sugar and 1/2 cup water. Bring to a simmer over medium heat, stirring until the sugar dissolves completely. Remove from heat and stir in 1/2 tsp almond extract. Let the syrup cool completely; you can even make this ahead and chill it.

In the bowl of a stand mixer fitted with the paddle attachment (or using a hand mixer), cream the softened butter and 1/2 cup granulated sugar on medium speed until light and fluffy, about 2-3 minutes. Scrape down the sides of the bowl.

Add the large egg and beat until well combined, scraping the bowl again. Stir in 1 cup almond flour, 2 tbsp all-purpose flour, 1 tsp almond extract, and a pinch of salt. Mix on low speed until just combined and smooth, taking care not to overmix.

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

Carefully slice each croissant horizontally through the middle, almost but not entirely separating the halves, creating a hinge.

Using a pastry brush, generously brush the cut inside surfaces of each croissant with the cooled almond syrup. Then, spread a thick layer (about 2-3 tablespoons) of frangipane filling onto the bottom half of each croissant.

Close the croissants. Generously brush the tops and sides of the assembled croissants with the remaining almond syrup. Spread a thin layer of frangipane over the top of each croissant (this will help the almonds stick).

Arrange the sliced almonds evenly over the frangipane on top of each croissant, pressing them gently to adhere.

Place the assembled croissants on the prepared baking sheet. Bake for 18-22 minutes, or until the frangipane is puffed and set, and the almonds are golden brown and toasted.

Remove from the oven and let cool on the baking sheet for a few minutes, then transfer to a wire rack to cool slightly. While still warm, dust generously with powdered sugar.

Serve warm and enjoy your homemade classic almond croissants!