In a small saucepan combine water, sugar, and almond extract.

Simmer over medium-low heat for 6 minutes, whisking often. Once thickened, remove from the heat and set aside.

In the bowl of a stand mixer fitted with a paddle attachment combine butter and sugar.

Beat together on medium speed for 2 minutes, until smooth.

Add in eggs and mix to combine. Add in almond flour, almond extract, and salt. Mix until incorporated.

Preheat the oven to 375 degrees F. Open all the croissants and place onto a parchment lined baking sheet.

Brush the inside of each half with a generous amount of syrup. Spoon some of the frangipane onto the bottom half of the croissant, about 1 Tablespoons worth.

Place the top half of the croissant on top, then spread more frangipane on top. Top with plenty of almond slivers.

Place the baking sheet into the preheated oven on the center rack. Bake for 18-20 minutes, or until crispy and golden brown.

Once baked, remove from the oven and place onto a wire cooling rack to cool down. Dust with powdered sugar if desired. Croissants are best served while warm.