Preheat an oven to 300°F. Have ready six ½-cup custard cups or ramekins and a shallow baking pan large enough to hold all the cups.

In a saucepan over medium-low heat, melt the butter. Add the brown sugar and, using a heatproof rubber spatula, stir to combine with the butter and cook until the sugar starts bubbling like molten lava, about 3 minutes.

Stir in the cream and let it bubble away, stirring with a big whisk until the mixture is smooth and slightly thickened, about 5 minutes. Stir in the milk.

Whisking constantly, slowly pour the egg yolks into the hot cream mixture. Stir in the vanilla, whiskey and salt. Pour the custard through a fine-mesh sieve into a pitcher.

Divide the custard evenly among the custard cups. Place the cups in the baking pan and carefully pour hot water into the pan to reach halfway up the sides of the cups.

Bake until the custards are set but still jiggle slightly when shaken, 25 to 30 minutes.

Carefully transfer the baking pan to a wire rack and let the custards cool slightly, then carefully lift the cups out of the water bath.

Let the custards cool on the wire rack for 20 minutes. Cover with plastic wrap pressed directly onto the surface of the custards to prevent a skin from forming.

Refrigerate until the custards are well chilled, about 4 hours or up to overnight.

Serve the custards cold with dollops of whipped cream.