Balsamic Vinegar Reduction Sauce

This is a quick and easy sauce to use for beef steaks and other beef cuts.

2 cups real aged balsamic vinegar, good quality (don't get the cheap stuff)
3 or 4 ounces cold butter, cut into chunks

In a small saucepan, bring the balsamic vinegar to a simmer and reduce by 2/3. One chunk at a time, swirl in the butter and combine until it's completely melted, and the sauce has a smooth, velvety consistency. Serve immediately over beef tenderloin or other red meat.