Preheat oven to 160°C. Line an oven tray with baking paper.
Place butter and sugar in a large bowl or bowl of your stand mixer. Use electric beaters to beat for about 4 minutes, until pale. Add egg, ground almonds and vanilla and beat a further minute to combine.
Slice croissants in half like a bun, leaving a hinge. Spread two heaped tablespoons of mixture inside each of the croissants. Then close the lid and spread 1 tablespoon of filling on the top of the croissant.
Pour sliced almonds onto a plate and then press the top of the croissant into the almonds, so they stick to the filling. Transfer to lined tray and bake for 15-20 minutes, until slightly golden and crisp.
Dust croissants with icing sugar and serve warm with a hot cup of coffee, à la français!