In Chilcoot, a ranching town of 450 near the Nevada border, Tim Kanavel, who was born on a ranch, and his wife, Carolyn, have run the Chilcoot Frosty for 15 years. Their faded hand-painted sign features a smiling cowboy pointing at a carton of fries. We stopped by one hot and hazy morning, under drifting smoke from a Yosemite wildfire. I yearned for something cool and creamy, so we diverged from our policy of eschewing soft-serve before noon. We split a fresh blueberry shake – and it hit the spot.

Farther south, the sign at the Sno-Flake Drive-In in touristy South Lake Tahoe is a laser-printed replica of the original, and there are three kinds of "sortaburgers" – veggie, salmon and turkey – to cater to the non-beef eaters that tend to migrate to resort towns.

Nearly all of the frosties we visited have been around since the early 1960s, and black-and-white photos on their walls show clean-cut couples licking soft serve in classic cars, polished chrome and fins shining. But other than their vintage, what distinguishes frosties from regular ice cream shops? Like pornography, frosties are easier to recognize than to define. In a way, you can tell a frosty by what it's not. Thus, Katie and I concluded that Treats, a boutique ice cream shop in bourgie Nevada City, in the Lost Sierra foothills, does not qualify: A single scoop of Treat's hard, hand-scooped ice cream cost $3.25 (flavors included lemon custard, green tea and bittersweet chocolate), and shining glass domes covered plates of gluten-free brownies. I looked skeptically at Katie, who was licking a scoop of (admittedly) very delicious mint chip. "We gotta get out of here," she said. "We're out of the frosty belt."

Two and a half hours later, we were back in the mountains, in Portola, another small town, eating kraut dogs, onion rings and tater tots at a picnic table outside the Portola Frosty. Its giant, peeling wooden sign featured a 5-foot-tall cone, a burger that towered over my head and, in red letters, the word FROSTY. The portions were as large as the customers' waistlines, and the greasy food hit our stomachs hard. "Are we going to get fat?" I wondered as we drove off looking for a place to camp. "We might need to eat salads for a week and a half after this," Katie said.

It was the experience of eating, rather than the food, that inspired us to become frosties tourists. There's something about entering a world where generations of people have sat outside underneath the same pine trees for at least 50 years, eating the same kind of burgers and shakes. The ambiance is what matters– the hand-lettered signs, warped wooden picnic tables and rotating cast of teenage girls behind the window. Even if the menu evolves to attract new customers – many frosties now serve dairy-and-gluten-free soft-serve, brown rice and vegetable stir-fry or veggie burgers – the emotional experience of eating remains the same.

I marveled how nearly every small town we visited could support a frosty. In the Sierra Valley, we hit four of them in 40 miles. Two were for sale, though, and only one, Rhonda's Lil' Frosty in Loyalton, appeared very busy. But regardless of how dietary trends and fears of obesity affect the business, I suspect nostalgia will keep customers coming back. It's hard to resist the menu at Bud's Jolly Kone in Red Bluff: a 50-cent hamburger with any purchase, plus a receipt printed with the words "All Good Things Come From The Father Above." Naturally, that includes frosties.

This story is part of an April 2014 special issue of the HCN magazine devoted to travel in the West. Emily Guerin is an HCN correspondent and radio producer based in Paonia, Colo.