American vintners have always taken the best grapes from everywhere else for their table wines, and it's the same with dessert winemaking. We have Sauvignon Blanc and Semillon dessert wines made from grapes rotted by the botrytis cinarea fungus (following France). We have ice wine and late-harvest Riesling (following Germany). We have sweet Muscats and Malvasias (borrowed from Italy and Spain).
Yet we also go Europe one better with dessert wines based on Zinfandel (which is not technically native to North America but might as well be). Zin turns out to have some of the best qualities of classic dessert wine grapes, including the ability to shrivel into raisins without losing precious flavors. The raisins have a much higher ratio of sugar to acid and water than normal ripe grapes do, so they produce sweeter, thicker wine.
Zin also stands up well to fortification with distilled spirits. The British began preserving red wine with brandy when wars with France cut off the normal supply of red wine. There was plenty of red wine available further south, in Portugal, but the longer sea voyage was hard on it. So in went the brandy, and out came what we now call Port.
Zin port is a lot like the Portuguese kind in beverage terms, but it comes without the weight of history, vintage rules, and import logistics that true Port carries. So it's generally less expensive but just as wonderful a companion on a cold winter night.
American versions of classic European dessert wines are equally absorbing these days. Fifty years ago, Myron and Alice Nightengale made botrytis dessert wines with Sauternes as their model - for taste, anyway. Their method was another story. Whereas French vintners had always left a portion of their fruit either on the vine or laying on mats in the vineyard where botrytis could find it, the Nightengales got some botrytis spores from Europe and sprayed them on grapes in a moist, warm greenhouse.
Today vintners in California can swing either way: taking naturally botrytized grapes in years when Nature gives them, and inducing the "noble rot" when it doesn't.
Growers also use high-tech tools to spot slower-ripening blocks in their vineyards and extended spells of good weather late in the harvest season. This combination can produce the grapes required for "late harvest" wines - including both whites (Chardonnay, Riesling, Sauvignon Blanc, Semillion) and reds (Cabernet, Syrah, Zinfandel).
Winemakers don't need rot for their late-harvest wines. They just need the skins of the grapes to thin enough so that the liquid inside begins to evaporate. Once again, this reduces water and acid while leaving the sugar, resulting in wine that is oh, so sweet.
It's not necessary to know how American dessert wines are made. It does help you understand why they cost so much, though. There's always an economic risk in leaving fruit out in the field past the normal harvest date, and it's tough, time-consuming work to turn raisins into wine. To offset these costs and risks, vintners make us pay a bit more for dessert wine. In fact, they always have.
Is it worth it? For the past four centuries, the answer has been a lipsmacking yes.