Whip yolks and sugar until light. Stir in milk, cream and whiskey.
Pour into 4 buttered ramekins. Bake in the preheated oven until set, about 1,5 hours (start checking early, the time depends on the depth of the ramekins).
Take out of the oven and allow to cool completely; then place in the frige for a few hours.
Just before serving, sprinkle brown or muscovado sugar on top of each cold crème brûlée. Caramelize the sugar by using a torch or putting under a broiler. Work quickly - sugar should caramelize but the crème brûlée should still be cold!