Bring the balsamic vinegar, stout, & sugar to a low simmer in a heavy-bottom saucepan over medium-high heat.
Stir occasionally for 20-25 minutes, or until the mixture is reduced to 1 cup & syrup-like in consistency. Set aside & allow to cool, until needed.
Meanwhile, drink remaining contents of opened stout beer. When finished, recyle your bottle, then open additional bottle of stout beer & savor that flavor.
Skewer on each toothpick the prepared tomatoes, mozzarella & basil.
-Garnish with olive oil, salt & pepper, to taste, & drizzling of balsamic stout reduction.