The strip malls of Chantilly may slow down a bit on Friday nights, but on this hot steamy evening there's a steady stream of activity in the middle of Sully Plaza. One by one, a parade of mini-vans and SUVs peel off Route 50 and Centreville Road and wend their way through the parking lot toward a clutch of tables topped with umbrellas and flanked by two oversized plastic cones.

But it's not ice cream that's drawing the crowds on this midsummer evening. It's frozen custard, or more specifically, Milwaukee Frozen Custard, a concoction that has the taste of ice cream but with a smooth dense texture that's, well, just plain creamier than ice cream.

"Where else would you see people who are happy while they stand in line, happy while they're eating and happy when they leave," says Al Casey, the enthusiastic Wisconsin native who's the brainchild behind this 18-month-old venture.

Most of the crowd lining up inside the brightly lit parlor probably doesn't even know what makes frozen custard different--unless they've been privy to Casey's infectious spiel.

You see, says Casey, both ice cream and frozen custard must contain at least 10 percent butter fat. But what separates custard from ice cream is the 1.4 percent egg yolk in the custard, and the fact that little air (or "overrun" in custard parlance) is pumped into the final product. "You can take a gallon of ice cream and turn it into four gallons just by pumping it with air," says Casey, who grew up in the city that's universally recognized as the frozen custard capital of the world. That would be Milwaukee, of course.

In fact, his super-secret custard mix comes directly from Milwaukee, delivered every two weeks by refrigerated truck and stored in a gigantic walk-in freezer in the back of the Chantilly shop. From there it's mixed in two "continuous-feed machines designed to work exactly the way a hand crank does," says Casey, who came up with the machine's design himself. "A motor turns the crank and we use freon instead of freezing water, but a dasher still scrapes down the sides, just like with an old-fashioned hand crank."

Casey's final product is never frozen, the way ice cream is. Instead, just enough custard is churned out of his two machines to meet the day--or the hour's--particular need. It's that churning that creates the chug-chug-chug sound you hear coming from the two stainless-steel hulks, which produce not only the standard vanilla and chocolate flavors, but also whatever special flavor is being highlighted. In a typical month, more than 15 flavors are offered. A month's worth of these flavors-of-the-day is printed up and available in the store (and on the shop's Web site at www.milwaukeefrozen; today it will be Reese's Cup-- "real Reese's Peanut Butter Cup in our vanilla custard").

Other specialties include something called "concrete" ($3.99), which is a custard flavor blended with any of the 18 different candy pieces known as "gravel"; a "creamy" ($3.99), which is Casey's name for a shake blended with one of six different fresh fruits; plus assorted sundaes ($3.50 each), banana splits ($4.49), malts ($3.75), shakes ($3.50) and floats ($3.25). Cones and dishes range in price from $1.49 for a child's portion to $2.99 for two scoops. Casey also sells custard pies ($9.99) and cakes ($15.95 to $29.95).

While plenty of the 300 or so customers who make the trek this Friday night will come and go quickly, lots of folks linger around the 2,000-square-foot shop, which, except for the tile floor, Casey built, wired and plumbed himself. The kids like to watch the two toy trains that circle the perimeter of the ceiling, look at pictures of happy customers or of some of the country's better known frozen custard joints, with names that conjure up images of '50s Americana: Kopp's, Culver's, LeDuc's and Town Pride, to name a few.

Casey, 49, a self-professed ice cream/custard "addict," has traveled from coast to coast in search of custard, which was first invented by the Kohr brothers in 1919. (As Casey tells it, the Kohrs added egg yolk, a natural emulsifier, in an attempt to keep their ice cream from melting so fast.) He's partial, of course, to the varieties sold in his home town of Milwaukee, but admits that he'll stop anywhere for a fix.

"I just love a cone," says Casey. "It's always festive, always social. You might fix yourself a dish of something at home, but never a cone."

For that you need to go out, to make something special out of an evening, whether it's a stroll down the boardwalk while on vacation, or just a quick trip to Chantilly.

Milwaukee Frozen Custard, 13934 Lee Jackson Memorial Hwy., Chantilly. Call 703-263-1920.

CAPTION: Milwaukee Frozen Custard's Al Casey enjoys his cone; behind him are photos of happy customers.