The Polka Dot squats at the edge of town, a vintage burger and soft-serve joint with a bright blue Chateau-style roof and a large primary-colored sign that proclaims: FROSTIES. Standing beneath the restaurant's massively precarious and rusty swamp cooler, you can choose between fried pickles and Texas Toothpicks (battered, deep-fried jalapenos and onions), a dipped cone or a fresh strawberry shake. Just after dinnertime on a warm August night, four local teenage girls in black shirts and tight jeans lean over the counter, bantering with a friend who works here. In the parking lot, customers in cowboy boots idle in huge trucks, slowly licking swirls of soft-serve.

This is Quincy, a town of 1,700 located 150 miles northeast of Sacramento, in the heart of the Lost Sierra, a part of California made famous by the gold-seeking '49ers. The steep forested hillsides plunge into clear creeks that still sparkle with flecks of gold. Local geological consultants stake and sell placer claims, and shops carry sieves and pans for people working the streams. But the Gold Rush isn't the only part of the past that lingers here: Deep in the Lost Sierra, the frosty still thrives.

The word "frosty" has two connotations: either the swirl of vanilla or chocolate soft-serve on a cone, or – my main interest – the classic establishments that dish it out. Typically, the restaurants date back to the 1960s and sell made-to-order fast food – corndogs, burgers and a lot of finger foods – for prices that have changed little over time.

Elsewhere, they're known by different names – shake shacks, dairy freezes, drive-ins – but in the Lost Sierra they are almost always called frosties. Last August my friend, Katie, and I hit nine of them on a three-day road trip. Why? For the whole retro experience, the distinctive culture, and something we realized as we roamed: The uncanny way that frosties mirror their clientele and their towns.

The dessert originated in the 1930s, when either the founders of Dairy Queen or (there is some debate) Carvel Ice Cream, which billed itself as "the nation's first retail ice cream franchise," discovered that customers loved ice cream served at a slightly warmer temperature than normal, right out of the machine. But it was Dave Thomas, Wendy's founder, who capitalized on the name, putting the 35-cent category-bending treat (was it a beverage, or a dessert?) on the original 1969 Wendy's menu. Today, frosties remain one of the chain's most popular items.

Wendy's frosties are served in a cup with a spoon, but the frosties of the Lost Sierra proudly retain the original form, spiraling skyward from cones, either naked or dipped in liquid chocolate that forms a thin shell. They yield easily to the teeth and dissolve swiftly on the tongue. Frosties are low-fat and airy, with a variety of unpalatable-sounding additives that keep them from melting too fast, growing ice crystals or getting chunky.

Frosties are everywhere in the Lost Sierra. We began our tour at the Pine Shack Frosty in Chester, Calif., a town of barely 2,000 that's a gateway to Lassen Volcanic National Park. The restaurant's menu was hand-lettered by a one-legged old man named Bubba, who has also painted most of the signs in town, lending the entire place a pre-digital feel. Your meal is on the house if Mount Lassen erupts while you order.