In a mixing bowl, cream together the butter and sugar until light and fluffy
Add the egg and mix until smooth
Stir in almond meal and almond extract until well combined
Preheat oven to 180°C (350°F)
Slice croissants in half lengthwise
Brush the inside of each croissant half with simple syrup
Spread a generous layer of almond filling inside each croissant
Close the croissants and spread a thin layer of filling on top
Sprinkle generously with sliced almonds
Place croissants on a lined baking tray
Bake for 15-18 minutes, or until golden brown and crisp on top
Allow to cool slightly
Dust with icing sugar if desired
Serve while still warm
