Almond croissants originally were day-old croissants that had almond cream (crème d’amandes) piped in them and that were then brushed with a simple syrup and sprinkled with sliced almonds, then baked again until the filling had set and the croissants had crisped up.
Now, they are purpose made.
The syrup protects them from burning.
They are very rich. Some can be very sweet, or have a good deal of artificial almond flavouring in them.