
Stout is a dark, heavy-bodied, top-fermented ale that takes its roast from dark-roasted barley or malt.
Porter
The closest swap. Use the same amount. Porter tends to taste rounder and more chocolatey, stout drier.
Another dry stout
Works too, including a non-alcoholic one such as Guinness 0.0. A nitrogenated stout gives the creamiest head.
Milk stout
It makes the drink sweeter.
The substyles matter more than the word. Dry Irish stout uses roasted unmalted barley for a coffee-like dryness, oatmeal stout adds oats for body, and an imperial runs far stronger and sweeter.
Opened beer loses its fizz quickly — use it fresh.