No city appears more intensely American than Williamsburg, Va. Not only do the historic buildings and the modern-day park service facilities look American to the core, so do the people -- the staff of the historic site in colonial dress and visitors in regulation tourist costume of T-shirts, Nikes and Instamatic-camera necklaces. And they fill themselves with all-American staples from brunswick stew to fast-food burgers.

So Le YaCa is a space warp -- the soul of France on the commercial strip linking Williamsburg with Busch Gardens. The proprietor, Daniele Bourderau, greets you with a warm lilting singsong of a voice that almost makes you believe you could answer her in perfect French. She looks like somebody's grandmother -- yours, you wish. And behind her is a tile fireplace with lamb revolving on a spit if it is evening, or a table of about 15 very French hors d'oeuvre salads if it is lunchtime. The tables, in a dining room with homey lamps and a glass-fronted sideboard, are set with lace cloths; you could almost imagine you should have brought a house gift rather than your credit card.

What in the world is such a French outpost doing in the middle of ultra-America?

Bourderau had two restaurants in France -- one near St. Tropez and another in the Alps -- when in 1968 she decided to look for an American hedge against the economic crisis she expected in France. Every year she would come to the United States to seek a location, visiting two or three states at a time. After 10 years of that she encountered childhood friends in Paris who had bought a plantation in Virginia and encouraged her to visit. She did, and fell in love with Williamsburg. Four months later she found her dream house and then opened in a little shopping center a traditional French country restaurant aptly named, like her remaining French restaurant near St. Tropez, Le YaCa, slang for "Let's do it."

As for operating a French restaurant in a colonial American atmosphere, "That's been very, very hard," said Bourderau. In the beginning she couldn't get any Williamsburg customers, she explained; only about 5 percent of Le YaCa's diners were from the immediate area. "It is finally the people from out of town who make the local people come," she said.

Finding kitchen staff was a problem, too. "That's been my nightmare," said Bourderau, shuddering and waving her hands and raising her eyes to the ceiling. In France she would get students from good schools and train them; in the U.S., she said, "When they get out of school they think they know everything, and they don't know much." Then when you have trained them, she added, "they think they are the greatest chef on earth and they go somewhere else." In the first three years she had nine chefs at Le Yaca, though she finally found one from France two years ago.

A third adjustment was, "It took me about three years to figure out what to avoid" in terms of food. Sweetbreads were removed from the menu -- though not from the kitchen. She now combines them with veal and doesn't mention their presence. "I've never had a piece left of sweetbread," she discovered. The menu, divided at dinner into fixed-price meals of four or eight courses ranging from $12 to $38.50 -- is traditional: loin of veal with mushroom cream sauce, scallops with chablis butter, seafoods in lobster sauce, duck breast with sweet-sour orange sauce, chateaubriand bordelaise. But it is her leg of lamb, spit-roasted over a wood fire, that is the hit of Le YaCa. It accounts for 45 percent of the dinner sales.

That is eclipsed only by Bourderau's irresistible dessert, marquise au chocolat, a recipe with a homey origin. It's been her restaurants' specialty for 22 years, ever since her daughter clipped it from an unknown magazine while she was sick and asked the chef to make it for her once she felt like eating again.

The chef made it for the restaurant as well, to his resounding success. There are a lot of recipes for marquise au chocolat, said Bourderau, but none as good as this one from its unknown source. Our waitress described it as a chocolate truffle formed into a loaf and sliced. My daughter responded to it with, "It's the first time I've fallen in love with a non-living thing." Tabletalk

It's increasingly hard to tell a cooking class from a vacation. The newest glamorous excursions in culinary education are: LaVarenne in Burgundy, beginning early next summer, week-long cooking-and-eating tours based at a 17th-century chateau, run by the Paris cooking school. For information contact La Varenne's Burgundy, P.O. Box 25574, Washington, D.C. 20007; (202) 337-0073. Chef Hubert, also of Paris, will teach on St. Barth's in the French Antilles this winter, in week-long sessions between Jan. 20 and Feb. 14. Write Pat Wright, 3770 Tansy St., San Diego, Calif. 92121. Or call (619) 453-8900. Drive-in restaurants may be giving way to drive-out restaurants. Burger King already has six mobile restaurants in Florida and four elsewhere in the U.S. Pizza Hut is also getting rolling with its Pizza-2-U vehicles. MARQUISE AU CHOCOLAT DU YACA (10 servings)

12 ounces European bittersweet chocolate

3/4 cup (1 1/2 sticks) butter

5 eggs

6 tablespoons sugar

Melt chocolate with butter over low heat until very smooth; set aside. Separate eggs. Whisk yolks with sugar until foamy and pale. In another bowl whisk the whites until stiff.

While the chocolate mixture is still lukewarm, mix it with the yolks. Then, with a wooden spoon, delicately blend in the egg whites.

Pour the preparation into a 5-cup buttered loaf pan and put it in a freezer for a minimum of 5 hours, covered with plastic. Take out of the freezer 1 hour prior to serving.

Serve it sliced on a plate covered with cre me anglaise. CREME ANGLAISE (Makes about 2 1/2 cups)

2 cups milk

1 1/2 teaspoons vanilla extract

6 egg yolks

6 tablespoons sugar

Scald milk and add vanilla. Beat the egg yolks and sugar together in mixing bowl until pale and the mixture forms a ribbon when the whisk is lifted.

In a heavy enameled saucepan, heat the egg mixture over low heat, stirring constantly with a wooden spoon. Gradually add the hot milk, continuing to stir. Continue to cook over low heat, stirring until the mixture thickens and will coat a metal spoon (about 165 degrees on a candy thermometer). The mixture must not be allowed to boil. Immediately remove from the heat and pour into a cold container. When thoroughly cool, cover the container and refrigerate until serving time.