Pour Guinness® Stout Beer (12 fluid ounce) into a medium saucepan and heat to boiling. Reduce heat to a rapid simmer and cook until the stout is reduced by half.
Stir in the Brown Sugar (3/4 cup) and Vanilla Extract (1 teaspoon). Heat to boiling, reduce heat to a rapid simmer and let the mixture cook 8-10 minutes without stirring until sauce turns a deep amber and bubbles start to grow and expand.
Remove from heat and add the Unsalted Butter (2 tablespoon), Heavy Cream (1 cup) and Salt (1/4 teaspoon) and stir carefully until fully combined.
Transfer to a resealable glass storage jar and let the mixture cool to room temperature.
As an option, spoon over ice cream and/or pound cake.