Leftover croissants can be made into almond croissants which is a great way of using up leftovers. Cream butter and sugar with almonds and flour until fluffy, and add the egg over three additions. Leftover croissants are cut in half, the crumb brushed with rum syrup, piped with almond cream, and the top is piped with more almond cream, then dipped into almond flakes. The almond croissants are baked then for 15−20 minutes at 180°C, allowed to cool and then dredged in icing sugar. Tip: if dusted when still warm, the icing sugar will turn yellow and does not look as attractive.
Full recipe and more tips available in my book, The Art of Lamination: Advanced Technical Laminated Pastry Production.