Before you start, preheat oven to 170°C.
Make filling. Combine butter and sugar in a bowl and whisk until smooth. Add vanilla and egg and mix well. Fold in ground almonds until fully incorporated.
Use a bread knife to slice croissants through the middle so you can open them up like a sandwich.
Fill croissants. Spoon 2 tablespoons of almond mixture onto the base of each croissant and spread out evenly to the edges, then close the lid. Spoon 1 tablespoon of almond mixture onto the tops of each croissant and spread out to the edges.
Top croissants. Place the sliced almonds onto a plate. Pick up each croissant and place them in the sliced almonds, top side down so they stick to the filling mixture. Press down firmly and squash slightly.
Place croissants on a lined oven tray and bake in oven for about 15 minutes, until golden and crispy.
Serve with a cup of coffee!