In 1681, the little town of Rust (pronounced “roost”) in what is now Austria’s Burgenland province became a “free city,” buying its freedom from Emperor Leopold I with 60,000 gold guilders and 500 buckets of Ruster Ausbruch, its finest sweet wine.

Centuries later, Ausbruch is still Rust’s finest sweet wine. Surrounded on three sides by an amphitheater of vineyard slopes, its fourth side a port on Lake Neusiedl (whose surface helps regulate climate conditions), and with 300 days of sunshine, Rust has an ideal setting for producing its Ausbruch.

To understand Ruster Ausbruch, consider its place among the world’s greatest naturally sweet wines. Trockenbeerenauslese, the sweetest category, must have a sugar content of 30 brix at harvest. Ausbruch must have 27 at harvest, while the notable Beerenauslese must have 25.

Only a few regions in the world have the right conditions for Botrytis cinerea, the noble fungus that gives us these wines. It attacks the grapes, extracting much of the juice and thus concentrating the natural sugar. By the time the shriveled grapes are picked individually and pressed, they may give only droplets of juice — sweet, rich, concentrated juice.

Made of any white grape approved for growing in Rust, either totally of one varietal or a blend of grapes, Ruster Ausbruch at its best is a harmonious, opulent wine offering an explosion of fruit flavors; a creamy, honeyed texture; finesse; and luscious sweetness balanced with a spine of vivid acidity. The best can live for decades.

Among the producers of Ruster Ausbruch at its highest quality are Heidi Schröck, Feiler-Artinger, Alois Kracher, Peter Schandl, Wenzel and Hopler.

Most people serve sweet wine only at the end of the meal. And Ruster Ausbruch is ideal for dessert, provided it’s not served with an overly sweet and competing dish. Ruster Ausbruch is also a tantalizing partner to blue cheese, foie gras and nuts.

Most Ruster Ausbruch is sold in 375 ml bottles, half a standard wine bottle. And because so little juice is extracted from each Botrytised grape, the price usually ranges from $69 to $110 and more for a special one. Expensive, yes, but a rare treat.