There are 3 categories of balsamic vinegar—traditional, condiment, and imitation. The one you want here is condiment or "condimento balsamico," identifiable by the I.G.P. (indicazione geografica protetta-protected geographical indentification) designation on the label. Condiment balsamic is the most widely used version of balsamic—it's still balsamic but not the strictly produced or expensive top-shelf stuff. It's the most versatile and you can get a decent bottle for a decent price. Darker condiment balsamic vinegars are sweeter and better for drizzling, just like in this recipe.