DURING A recent visit to San Juan, I was honored by an invitation to lunch by the First Lady of Puerto Rico, Senora Kate de Romero, the wife of Governor Carlos Romero-Barcelo. It was held in the white, Spanish-style, fortress -- Palace La Fortaleza, where they live and entertain. In a small drawing room, Kate greeted me with offerings of nonalcoholic tropical fruit punch and a fascinating array of hot and cold canapes.

Kate Donnelly was born and raised in Baldwin, Long Island. During her first secretarial job in New York, she vacationed in Puerto Rico. For about 10 days, she had such a wonderful time -- especially with the food -- that the moment she got home, she began sending out the first of about 40 job application letters to possible employers in San Juan. Within a few weeks, even before she knew any Spanish, she accepted a job with a New York bank at its San Juan branch. She and Romero-Barcelo were married in 1966.

"I guess food was one of the very important factors in my relationship with my husband," Kate said. "He so much enjoyed the authentic native dishes of Puerto Rico -- dishes that are often a complex and wonderful blend of African, Indian and Spanish influences -- that I knocked myself out trying to learn quickly how to prepare them."

Kate has become integrated into the cultural, political and social life of Puerto Rico. She now speaks fluent Spanish and, almost every day, pays an official visit to some town or village, lending her support to youth groups and the service programs for the aged and the handicapped. "The lovely thing about my work," she told me, "is that, everywhere, I receive gifts of homemade foods: cakes, cookies, traditional casseroles, country concoctions from old family recipes . . . We bring them all back to La Fortaleza, where they are put into a special refrigerator known as 'Kate's Secret Hideaway.' No one is allowed to touch anything in there until I have tasted it and released it. In this way, almost every day, I learn something new about the art and traditions of our Puerto Rican cooking."

Just before moving to the dining room for lunch, Kate took me into the very modern kitchen to meet executive chef Jesus Villalba. Whether it is a state dinner for 50, or a lunch for six, Kate believes for 50, or a lunch for six, Kate believes that every menu at La Fortaleza must be a a reflection of the culture of Puerto Rico. No concessions are made for foreign visitors. Prince Phillip of Britain lunched entirely on Puerto Rican specialties.

My lunch was in honor of the 30th anniversary of the beginning of efficiently organized tourism to Puerto Rico. It was a simple, perfectly balanced menu. There was a PERNIL, a roasted, unsmoked ham of fresh pork, filled and garnished with green olives and red peppers, served with a sauce of fresh sweet mangos from Mayaguez. The vegetable accompaniments were white rice with pigeon peas and cocoanut cream, a Puerto Rican cazuela (a casserole of baked sweet potato and pumpkin flavored with anise, cinnamon, clove and ginger), plus plantanos maduros, butter-sauteed plantains in a syrup delicately touched with rum.

But the top of that lunch, and to my taste the most unusual experience of that menu, was the second of three desserts. It was featherlight, small souffle balls of delicately baked, subtly sweetened egg yolks, covered with a coating of caramelized sugar and a crunchy crust of finely flaked fresh coconut. The name of the dish is yemas reales, royal egg yolks, and the appearance on the serving dish was, indeed, quite regal. CHEF JESUS VILLALBA'S YEMAS REALES (Royal Egg Yolks) (4 to 6 servings) 2 tablespoons or so of butter, for greasing baking pan and aluminum foil 2 large whole eggs 10 large egg yolks 1 teaspoon baking powder 2 1/2 teaspoons cornstarch 1/2 teaspoon salt 2 cups brown sugar Zest of 1 small lemon or lime, finely minced About 1 cup finely flaked or grated coconut meat

Average time required: About 10 minutes advance preparation, plus about 25 minutes for baking the eggs, plus about a final 30 minutes for the sugar syrup, the poaching, decorating and serving.

Preheat your oven to 300 degrees. Liberally butter a 12-by-8-by-1 1/2-inch baking pan of heatproof glass, ceramic or earthenware -- not metal. Then line it with brown, wax or parchment paper and liberally butter this. Hold at room temperature. Beat the whole eggs and the yolks with an electric beater at high speed for 1 minute. Reduce the speed to slow, sprinkle in the baking powder, cornstarch and salt. Turn the speed back up to high and beat continuously for 5 minutes. Now pour this frothy, thickish mixture on top of the paper. Slide into the center of the oven and bake until the eggs are just set through, but still quite soft -- usually in 20 to 25 minutes. e

Put into a 1 1/2-quart saucepan the sugar and half a cup of cold water, and the chopped lemon or lime peel. Boil it all fairly hard, stirring it regualarly, to make a light sugar syrup. You will have reached the right point when virtually all bubbling stops and a candy thermometer shows a temperature of 218 degrees. (In candy making, this is called the "large thread" stage.) Adjust the heat so that the syrup holds at this temperature.

As soon as the eggs are just set, turn them out onto a board and peel off the paper. You now have a softly solid, flat "egg cake." With a serrated knife, cut the cake into about 28 squares. Very lightly, with your fingers, reshape each of these squares into a rough ball. Don't work at them too much. rDon't feel you have to shape each perfectly.

Now, using a spatula and slotted spoon, gently lower these yolk balls, batch by batch, into the simmering sugar syrup. for each batch, put in as many as the width of the saucepan will hold, without the balls touching will hold, without the balls touching each other. You may be able to put in 4 or 6 or 8 at a time. Each ball should poach in the syrup for exactly 1 minute, then be lifted out with the slotted spoon and placed on a large, buttered plate. As each poached ball becomes cool enough to handle, roll it lightly in coconut flakes. Arrange on a lightly buttered serving platter.

Turn up the heat under the remaining sugar syrup in the saucepan and boil it fairly hard to the "large pearl" stage -- with the candy thermometer going up to 222 degrees. At this point, dribble a small amount of the hot syrup evenly over the balls on the platter. The syrup will cover the white coconut with a deep, rich, brown-gold glaze. Let the yemas reales cool, then serve at room temperature.