Pre-heat oven to 180 degrees Celsius (20 degrees lower for fan forced).
Start by cutting the chocolate croissants in half, ensuring they cover the entire bottom of an oven-proof dish (one layer only). Use the butter/margarine to both grease the dish and spread on the pieces of chocolate croissants targetting the sliced ends. Arrange them in the bottom of the dish.
Whisk the eggs in a mixing bowl, then add the milk, cream, sugar and lemon zest before whisking well. Pour the mixture slowly over the chocolate croissants. If the pieces float to the top then press them down gently until they absorb the mixture well (the pieces may need to be pricked with a fork to speed this process up).
Add the sultanas (if using) to the top and sides of the pudding. Grate about 1/4 whole nutmeg over the pudding before placing (uncovered) in the middle of the oven for 20-25 minutes or until the top is brown. Serve immediately!