Start with the caramel - pour the sugar and water in a form, put it on a heated stove and melt to the desired color of the caramel. Leave it to harden and cool fully, then smear the form evenly with butter.
In a deep bowl, beat the eggs with the sugar using a mixer, add the vanilla and then the cold milk in a thin trickle. Tear the panettone into small bites in the form and pour on the cream slowly and evenly.
Place the form in another, wider, container with cold water, turn the oven up to 302°F (150 °C) and bake about 1 hour until the cream is ready.
Turn the oven off and leave the container in it, without taking it out, until cooled a bit. Invert the creme caramel in a suitable plate and leave to cool completely. Serve cold.