Special equipment: 4 (4-ounce) cups or ramekins
Preheat the oven to 325 degrees F.
Whisk the eggs in a bowl. Add the brown sugar, and mix well.
Place the milk, cream and orange zest, if using, into a saucepan and bring up to a simmer. Remove the saucepan from the heat, and gradually pour into the egg and sugar mixture, whisking continuously. Pour the custard mixture through a sieve and into a large jug, and skim off any froth from the surface.
Place the cups or ramekins in a baking dish, or wide shallow pan, and fill them up with the custard. Pour enough hot water into the baking dish to come halfway up the sides of the cups or ramekins (known as a bain-marie) and cover with foil. Bake in the oven for about 40 minutes, or until the custards are just set in the center.
Remove the custards from the baking dish, and serve warm or allow to cool. Either way, sprinkle a little bit of extra brown sugar on top just before serving.
If you wish, use a kitchen blow torch to caramelize, or 'brulee', the brown sugar on top for a lovely crunchy surface.