Cut the croissants in half horizontally, using a serrated knife. Put the bases on a baking tray.
Arrange the banana slices on top of the croissant bases. Top with the chocolate squares and cover with the croissant lids.
Bake in a preheated oven, 230°C (450°F), Gas Mark 8, for 3 minutes until the croissants are warmed through and the chocolate softened.