Keep your remaining ingredients close by. In a medium saucepan on medium heat, add the sugar and water. Allow to simmer on medium swirling gently every few minutes. Make sure not to use any utensils to stir the pan (this will prevent the caramel from forming) Any sugar crystals on the edges will eventually dissolve as they melt.
After about 5 minutes the mixture will turn from a clear smooth liquid, to sandy clear mixture, to a clear orangey-brown. Watch it carefully! DO NOT step away from the pan. Once the colour change starts it happens fast and can burn very easily! As soon as it turns a light amber colour take it off the heat.
Immediately add the cream and milk. Be careful! It will sputter and rise up a lot! Using a whisk keep stirring until the mixture calms down and the caramel is mixed into the cream.
Add the vanilla essence, if using, and slowly pour in the beaten eggs. Make sure to go slow here, you don't want the eggs to scramble in the hot liquid. Add a pinch of salt and stir to combine.
Pour the custard mixture over the prepared croissants, making sure the croissants are fully soaked in the custard. Sprinkle with slivered almonds, if using.
Bake for 20 minutes or until heated through and the pudding is cooked through and bubbling.