Preheat the oven to 160° C w/ fan.
Cut the panettone into 2 halves; set aside 1 half of the panettone for whatever you like - cut the 2nd half in thick slices (about 1,5 - 2 cm thick).
Butter the slices generously.
Butter the casserole generously.
Grate the orange zest.
Mix together the eggs, the milk & the cream w/ an egg whip.
Add caster sugar, vanilla sugar & orange zest.
Mix well.
Take a slice of panettone & dip it into the liquid mix (for 1-2 seconds or so) & place it in the casserole.
Proceed alike w/ all the panettone slices & fill the casserole.
Pour the rest of the liquid in the casserole.
Sprinkle brown sugar all over the top.
Let it bake in the preheated oven for about 40 min.