This butterscotch mousse is intensely butterscotchy and as light as air. The combination of buttery and caramel notes and the light texture makes for an individual mousse it's hard to stop eating.
Made with egg yolks, whipped whites, and whipped cream, it is made with the same components as a classic mousse, but the eggs are all cooked for food safety.
Whisk the egg yolks, cornstarch, and about 1/4 cup of the milk together in a bowl. Set aside.
Melt the butter in a pan.
Add the brown sugar, salt, and a little water so the sugar completely dissolves.
Bring all to a boil and cook, stirring constantly, until the butter browns and the sugar caramelizes (NOTE: The ratio of butter to sugar is such that the butter will separate out at some point. Don't worry. Trust your nose. When you smell caramelized sugar, you'll know it's done.
Once the sugar has caramelized, turn off the heat and pour in all the remaining milk.
The butterscotch may seize up. That's okay.
Cook over medium-low heat, stirring constantly, until the butterscotch has all melted.
Temper the hot butterscotch mixture into the yolk mixture, then pour everything back into the pan.
Bring to a boil, stirring constantly, and keep at a low boil for 2 minutes.
Strain into a bowl, and whisk in the vanilla.
Press plastic wrap onto the surface of the custard and refrigerate at least room temperature. It does not have to be completely chilled, but you don't want it to be warm, either.
For the Meringue
Put the egg whites, sugar, and salt into a metal bowl (preferably the mixer bowl from your stand mixer, but if you are using a hand mixer, any metal bowl will do as long as it will fit on top of a pot on the stove).
Set the bowl over a pot with 1" of water in it. This is. Turn the heat to high, and whisk the whites vigorously and continuously.
Once the water in the pot boils, control the heat to keep it at a high simmer, and continue whisking until the temperature of your whites is 180F.
Remove the bowl from the pot and whisk the whites, either on your stand mixer fitted with the whip attachment or with your hand mixer, until the meringue is thick and shiny and at room temperature. It will have tripled in volume. You have just made a brown sugar Swiss meringue.
Remove the bowl of custard from the fridge and whisk it a bit to loosen it.
Whisk in 1/3 of the meringue, and then fold in the rest, gently but thoroughly, until no streaks remain.
For the Cream and to Finish
Whip the cream to medium peaks along with the pinch of salt.
Fold thoroughly but gently into the rest of the mousse.
Ladle into serving glasses, cups, or bowls. You can also scoop the mousse into a decorating bag fitted with a large star tip and pipe it decoratively into your serving glasses.
Chill at least 4 hours. For best flavor, allow to sit at room temperature for 45 minutes before serving with more whipped cream on top. Finely chopped toffee or chopped spiced nuts would also be a lovely garnish.
Notes
Refrigerate for up to 3 days or freeze for up to 2 months.