In a small sauce pan, over medium heat, cook the balsamic and sugar down until it is reduced by half and turns into a thicker syrup. But be careful, and don't walk away while it cooks. If it reduces too much, or gets too thick, it can scorch and the sauce will be ruined. Take it off the heat and add the soy sauce, more or less to taste.

Right before serving, at the very last minute, heat the sauce again and add the butter and whisk it together until it froths a little bit. Take it off the heat and drizzle over whatever you are using, whether it is fish, veggies, chicken or beef.