Rich, toasty, and malty thanks to the addition of vanilla porter, this sauce is addictive enough to eat straight from the jar...but it's even better on top of gooey, fudgy, crackly-topped brownies and vanilla ice cream. I've become a fan of the dish as a dinner party dessert, both because you can make everything in advance (assembling the sundaes when you're ready) and because people get pretty excited when you plop one down in front of them. Don't skip the cacao nibs—the slightly bitter crunch really balances out the other elements, pushing the dessert from nostalgia territory to something grown up and new.
Melt the butter in a saucepan over medium heat. Add the sugar, beer, salt, and cream and stir to combine.
Bring to a simmer over medium/medium-low heat. After about 3 minutes, it will start to simmer/bubble slightly, then puff up and get foamy, like somebody shook a can of coke. Continue to simmer, stirring every so often.
After about 10 to 12 minutes, the character of the bubbles will change slightly; they will be thicker, larger, and much darker in color, no longer looking foamy. Start stirring more frequently and watching your pot at this stage. Sneak a taste and add more salt, if you like.
Continue to simmer until the mixture is a deep golden brown and has been reduced by half, to 1 1/3 cup. (The whole process should take between 15 to 20 minutes, depending on how high your heat is and your patience level.)
Remove from the heat and stir in the vanilla extract. Let cool slightly before pouring over ice cream, French toast, apple slices, peach pie, brownie sundaes…whatever you like. It will keep in the fridge for about 2 weeks. Reheat slightly before serving.
For Each Brownie Sundae:
In a bowl or on a plate, top the brownie with a few scoops of vanilla ice cream. Drizzle or pour the Vanilla Porter Butterscotch Sauce over top and sprinkle with cacao nibs. Garnish with mint leaves if you're using. Repeat for everyone who wants a sundae (a typical brownie recipe makes 9).