Mix softened butter and sugar until creamy. Stir in egg and bitter almond flavour one after the other. Mix ground almonds and flour and stir in.
Preheat oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer). Line a tray with baking paper. Cut croissants lengthwise. Fill with some almond cream and spread the rest on the croissants. Sprinkle with almond flakes and bake in the oven for 10-15 minutes. Dust with icing sugar.
Croissants from the previous day can also be used.