Select croissants with a filling of your choice or with no filling at all. Cut them to same sized circles and arrange them in a cake pan or tray. Pour a mixture of beaten together eggs, milk, sugar and vanilla over them.
Put the croissant cake to bake in a water bath until ready. Put it in a cold oven to heat slowly at first and bake until nicely tightened (about 30-40 minutes).
Invert the cake into a tray once cooled. Serve with whipped cream if desired.