To make the frangipane, combine butter, sugar, and orange zest in a bowl of a stand mixer fitted with the paddle attachment. Beat on medium speed until pale and creamy, about 3 minutes.

Add the egg and whisk until it's fully incorporated, about 1 minute.

Decrease the mixer to low speed and beat in the almond meal. Scrape down the sides of the bowl to make sure all the ingredients are well incorporated.

Transfer the frangipane into a piping bag fitted with a flat piping tip. Refrigerate until ready to use.

To make a simple syrup, place water and sugar a small saucepan and bring to a boil. Lower heat to medium-low and whisk occasionally until the sugar has dissolved, about 10 minutes. Remove from heat and set aside.

Preheat oven to 350 degrees F. Line a large rimmed baking sheet with parchment paper and set aside.

Cut the croissants horizontally without cutting through.

Use a pastry brush to brush the cut sides of the croissant halves liberally with the simple syrup.

Pipe the bottom half of the croissant with 2 tablespoons of frangipane, creating even piping all the way to the edges.

Pipe the top with 1 tablespoon of frangipane. Top with 2 tablespoons of sliced almonds.

Place croissants on lined baking sheet and bake for 20-22 minutes until the almonds are toasted and the frangipane becomes golden brown.

Remove them from heat. Let them cool for 10 minutes before dusting them with confectioners' sugar and serving.