Boil the ingredients for the simple syrup for 3 - 4 mins and keep aside to cool to room temperature.
In a medium-size mixing bowl, whisk together the almond flour, all-purpose flour, and salt.
Mix in the butter using an electric beater or wooden spoon. Followed by the egg, vanilla extract, and almond essence. Scrape down the bowl using a spatula, beat until everything is well incorporated.
Refrigerate the almond cream for a minimum of 1 hour.
Slice the croissants horizontally in half.
Quickly dip all the slices in the cooled simple syrup, draining away the excess liquid, and place on a baking sheet lined with parchment paper.
Spread approx. 3 tbsp of the almond cream mixture on the bottom half of the croissant and 1 tbsp of the cream on the top of the croissant.
Cover the top with slivered almonds.
Bake in a 325F preheated oven for 16 - 20 mins or until almonds begin to brown on the top. Let cool about 5-10 minutes and then serve.
Dust with powdered sugar before serving.