This dessert is really pretty easy and simple to make. First of all, you would need to make the butterscotch by melting the butter over low heat and adding in the brown sugar, cook until the sugar starts to bubble, then the cream is added in. Stir until the mixture is smooth and slightly thickened, then pour in the milk. Stirring constantly, slowly pour in the lightly beaten egg yolks. Add in the vanilla extract, whisky and salt. Pour the custard mixture through a fine-mesh strainer into a pitcher.
Divide the mixture evenly among the custard cups, place in a baking tray, and fill the tray with hot water to reach halfway up the sides of the custard cups. Bake for 25-30 minutes, until the custard is set and will still jiggle slightly when the cups are lightly shaken. Let the custard cool slightly, remove the cups from the water bath, let cool on wire rack for about 20 minutes. Cover the top of each custard by pressing a piece of plastic directly on top of the custard to prevent a skin from forming. Chill the custards for a minimum of 4 hours or overnight.