A few years ago, I was driving through Chantilly, Va., a suburb of Washington, when a sign in a shopping plaza caught my eye. It read, “Milwaukee Frozen Custard.”

I immediately hit the brakes, crossed three lanes of traffic, and did my best to ignore the barrage of car horns and expletives. A few minutes later, I was enjoying a massive cone of America’s most sumptuous dairy product, my death-defying traffic maneuver a fast-fading memory. Frozen custard can have that kind of effect on people. If you’re among the relatively few New Yorkers who’ve feasted on this rarefied, dairyfied ambrosia, then you understand; if not, read on, for custard’s tale is one not often told in these parts.

Let’s start with terminology. “Custard” may make you think of pudding or tapioca, and “frozen custard” has become something of a generic catchall term for any Carvel-style soft-serve. But true frozen custard is a distinct product: By federal law, it must have at least 10% butterfat, at least 1.4% egg-yolk solids, and very little air.

Unlike soft-serve, which is dispensed fairly quickly and emerges in that familiar sculpted column, frozen custard is extruded very slowly in a long, flat strip, sort of like a giant tongue. It emerges from a specialized machine, popularly referred to as an iron lung. The extruded product is stored in a dipping bucket at about 18 degrees Fahrenheit (much warmer than traditional ice cream, which is typically stored well below zero) and, ideally, is sold at the same place as it’s produced, so it’s fresh each day.

These factors combine to produce an addictively satisfying treat. The high butterfat and egg yolks give frozen custard an incomparable richness. Unlike regular ice cream, it’s soft and creamy, but unlike soft-serve or gelato, it’s spectacularly dense and heavy. And because it’s served warmer than traditional premium ice creams, it doesn’t numb your mouth, so you can really taste the flavors. The higher temperature also provides a heightened olfactory element – you can smell those chocolate and vanilla notes before they hit your tongue. The net result, when properly executed, is like ice cream raised to an exponential power.

Frozen custard’s origins lie somewhere between history and folklore, but most authorities agree that it was born in 1919 on the Coney Island boardwalk, where Archie Kohr and his brothers were selling an early version of soft-serve on a hot summer day. Worried that their product might turn to mush in the sun, they added eggs to their recipe as an emulsifier, and frozen custard was born.

Kohr Bros. eventually developed into a chain, which now operates more than 100 outlets in 10 states. But they have no shops in New York City, which mirrors the larger paucity of frozen custard in its city of origin. These days, custard is primarily a Midwestern phenomenon (there’s a state-by-state breakdown at www.custardlist.com) and is especially popular in Wisconsin, where it’s the ultimate expression of the state’s identity as America’s Dairyland.

New York’s custard shortage may be rooted in economics, because selling custard is trickier than running a traditional ice cream shop. First, you need a few iron lungs, which can run about $50,000 each. Then you need very high-quality ingredients. And because custard has much less air than other frozen desserts, you’re selling a higher ratio of those ingredients with each serving (ice cream is sold by volume, not by weight, so fluffing up your product with more air, as soft serve vendors do, also puffs up your profit margin).

Moreover, custard’s primary attributes diminish with long-term storage, which means you can’t just make a huge batch and sell it over the course of a week – you have to constantly manufacture and sell at roughly equal rates. This in turn makes it impossible to have a large roster of flavors – the typical Wisconsin shop offers only chocolate, vanilla, and a flavor of the day. Toss in New York’s real estate costs and custard’s built-in price ceiling (nobody’s going to pay $10 for a cone, no matter how good it is) and you’ve got a daunting challenge.

All this may explain why there are currently only two custard shops in New York. One is Custard Beach, at the World Financial Center (street level, near the Winter Garden, 212-786-4707), the last of what was once a trio of Custard Beach outlets in the city. If the WFC shop is any indication, it’s easy to see why the other two places closed: The custard isn’t particularly rich, and it’s not dense at all – the vanilla is barely a notch better than Carvel. If this were your first frozen custard experience, you’d wonder what all the fuss was about.

The situation is better at the Shake Shack (in Madison Square Park, Fifth Avenue and 23rd Street, 212-889-6600), restaurateur Danny Meyer’s homage to Ted Drewes, the iconic custard stand of his native St. Louis. Just as Mr. Meyer’s Blue Smoke restaurant feels more like a curated version of regional barbecue than the real thing, the Shake Shack’s custard falls a bit short of its Midwestern inspiration. But it’s a quantum leap above Custard Beach, and is a good starting point for custard novices.

In classic Midwestern fashion, the Shack serves only vanilla, chocolate (which on a recent afternoon was dense enough to break my plastic spoon), and a flavor du jour, the best of which is mocha nut fudge, available on Wednesdays. They’re also offering something called Code Blue, a killer combination of vanilla custard and hunks of fresh blueberry pie.

But I’m still waiting for Midwestern-quality frozen custard to arrive in New York. And I’m sure it can be done, because that traffic-snarling custard I had in Virginia was the equal of any I’ve had in Wisconsin. If Virginians can do it, surely someone in New York should be able to as well, no? After all, we invented it.