In a microwave-safe bowl, combine the sugar and water. Microwave in 30-second intervals, stirring between each, until the sugar is fully dissolved. Stir in the almond extract and set aside to cool.

In a large bowl, beat the softened butter and granulated sugar with an electric mixer on medium speed until light and fluffy. Scrape the bottom and sides of the bowl, then beat in the almond flour until well blended. Add the egg, yolk, almond extract, vanilla, and milk. Beat until smooth and well combined. Set aside.

Preheat oven to 350°F and line a baking sheet with parchment paper.

Cut the croissants in half horizontally to create a pocket for the filling. Brush the inside of each side generously with the simple syrup. Spread or pipe a generous amount of almond filling on the bottom half of each croissant, leaving a small border. Add a layer of chopped chocolate, then place the tops back on.

Pipe a line of the remaining almond filling along the top of each croissant and press the sliced almonds into the filling. Arrange on the baking sheet with a little space between each.

Bake for 12-16 minutes until the filling is set and the croissants are golden brown. Remove from the oven and set aside to cool for 10-15 minutes.

While the croissants cool, melt the remaining chocolate in 20-second intervals in the microwave. Drizzle over the cooled croissants and dust with powdered sugar before serving.