If you managed not to immediately consume all the croissants you've made and have some leftover, this is a great way to use them to create an equally, if not more, delicious pastry. If you're deliberately saving some croissants for this, I would choose the runts of the litter since you will barely be able to see the croissant underneath the almonds anyways. This could probably work on non-homemade croissants, too, but you might as well just buy almond croissants then.
Almond Croissants (adapted from Bouchon Bakery)
73 g almond meal
7 g all-purpose flour
73 g unsalted butter, at room temperature
73 g powdered sugar
1 large egg
75 g sugar
75 g water
15 g almond meal
6 day-old traditional croissants
2 1/4 cups sliced blanched almonds
For the almond cream: Whisk almond meal and flour together in a small bowl.
Place the butter in the bowl of a stand mixer with the paddle attachment and mix on medium-low until the butter is the consistency of mayonnaise and holds a peak when the paddle is lifted. Sift in the powdered sugar and mix on the lowest speed until incorporated, then increase the speed to low and mix until fluffy, about 2-3 minutes. Add the almond mixture in 2 additions, pulsing to combine and then mixing on low speed for 15-30 seconds after each one.
Add the egg and mix on low speed until combined and smooth, about 30 seconds. Transfer to a covered container and press a piece of plastic wrap against the surface to prevent a skin from forming. Refrigerate until cold, about 2 hours. The almond cream can be kept in the fridge for up to 4 days.
For the almond syrup: Combine the sugar, water, and almond meal in a small saucepan and bring to a simmer over medium heat, stirring to dissolve the sugar. Remove from heat and let steep 1 hour. Strain into a small bowl. The almond syrup can be kept in a covered container in the fridge for up to 1 month.
Preheat the oven to 350°F. Line a baking sheet with a Silpat or parchment paper. Spread the sliced almonds out in a shallow bowl or dish.
Cut the croissants in half horizontally.
Liberally brush each cut side with the almond syrup.
Spread about 2 tablespoons of the almond cream onto the bottom half of a croissant. Place the top half back on and gently press.
Spread about 1 1/2 tablespoons almond cream on the top of the croissant.
Dip the tops of the croissant into the almonds, coating the almond cream generously, and press down to compact. Arrange the croissants on the prepared baking sheet.
Bake for 18-20 minutes, until the nuts are golden brown.
Set the baking sheet on a rack and cool completely.
These are best eaten the day they are made but can be stored in a covered container for up to 1 day.