Filling

1. Using a stand mixer fitted with a flat or whisk attachment, cream the butter and sugar in a mixing bowl at high speed for 1 minute or until well combined.  Alternatively, you can use a hand mixer. 
2.  Add the egg and almond extract to the butter-sugar mixture.  Beat on medium until well combined.  Scrape down the sides of the bowl halfway through mixing.
3. On low speed, add the almond flour until well combined.  Scrape down the sides of the bowl and mix for 30 seconds.  You may see pieces of butter in the filling,  that's okay.

Syrup

1. In a small microwave-safe bowl, combine 1 tablespoon of sugar with the water.  Heat in microwave on high for 15-20 seconds until the water is hot and the sugar has dissolved. Add the almond extract.  

Assembly

1. Preheat oven to 350°F.
2. Line a half-size sheet pan (13 inches by 18 inches) with parchment paper.
3. Slice the croissants in half lengthwise with a serrated bread knife. 
4.  With a pastry brush, lightly brush the cut sides with the croissants with the syrup mixture.
5. Spread 2-3 tablespoons of the filling on the bottom half of the croissant.
6. Place the top half on the filling.
7. Spread a thin layer of the filling on the top of the croissant.
8. Sprinkle the tops with 1 1/2 teaspoons of  sliced almonds.
9. Bake 8-10 minutes until they are lightly brown.  Times may vary depending on the accuracy of the oven temperature.
10. Remove from the oven.  Dust the croissants with powdered sugar.  Allow the croissants to cool slightly before serving.