1 to 2 days before baking bread pudding, allow croissants to become stale.
When ready to bake bread pudding, prepare almond paste. Combine almond flour, confectioner’s sugar, egg white, butter, almond extract, vanilla extract, and salt in a bowl. Beat with an electric mixer until smooth.
Slice croissants in half horizontally, fill with almond paste, and close. Chop filled croissants into pieces, place in a bowl, and set aside. Reserve any leftover paste for later use.
For custard, combine half-and-half, sugar, vanilla, and eggs in a bowl.
Pour custard over croissant pieces, making sure each piece is fully coated. Set aside to soak for 30 minutes. This step is very important to prevent egg custard from sinking to the bottom during baking.
Preheat the oven to 350 degrees F (180 degrees C) and grease an 8×8-inch baking dish.
Add croissant pieces to the prepared dish and top with remaining almond paste. Scatter almond slices over the almond paste. Cover dish with foil.
Bake in the preheated oven for 30 minutes, then remove foil and bake for 10 minutes more uncovered.
Remove from the oven. Allow bread pudding to cool for 10 to 15 minutes. Dust top with confectioner’s sugar, cut into 9 squares, and enjoy while warm.