Sift almond flour and all-purpose flour together in a medium bowl to remove any lumps. Set aside.

Add butter to bowl of a stand mixer fitted with a paddle attachment. Beat until butter is creamy and fluffy, about 2 minutes. Add powdered sugar and beat on low until combined. Increase speed to medium-low and beat until fluffy, about 2 minutes.

Add half of the almond mixture to butter mixture. Mix until combined. Add second half of almond mixture and mix until combined. Scrape the sides of the bowl to ensure even mixing. Add egg and mix on medium-low until completely combined.

Place mixture in bowl and cover with plastic wrap so that the plastic is touching the almond cream. Refrigerate for 2 hours.

With about an hour left for the almond cream to chill, begin the almond syrup. Add sugar, water, and almond flour to a medium sauce pan. Heat over medium heat and stir until the sugar has dissolved, about 3-5 minutes. Do not heat much longer than that or else the sugar will harden once cooled. Remove from heat.

Let almond syrup cool down and thicken for about 1 hour. Once cooled, add almond extract and stir to combine

Preheat oven to 350 degrees. Prepare baking sheet with parchment paper.

Slice croissants in half horizontally. Split open into two halves. Brush each half with a little almond syrup. Remove almond cream from refrigerator and split into two halves. Take one half of almond cream and split among the 4 bottom halves of the croissants. Spread with the back of a spoon to coat the sliced croissant evenly. Place croissant top onto each bottom to create a sandwich.

Take the remaining almond cream and split among the 4 croissant tops. Make sure to place on the top of the croissant, not the sliced part. Spread evenly among the croissant.

Place sliced almond into a shallow dish. Place the croissant into the almonds, pastry cream side down and press to cover the croissant in almonds. Repeat with other croissants. Place croissants onto prepared baking sheet.

Bake croissants for 18-20 minutes, or until almonds begin to brown on the top. Let cool about 5-10 minutes and then serve. Best eaten on same day.