These are some of the culinary secrets to emerge from the latest meeting of the select club of chefs who cook for the planet’s most powerful leaders. The “club des chefs des chefs,” a group of 27 top chefs from the kitchens of the world’s presidents, prime ministers, and monarchs, is gathering in Paris this week to swap recipes and tidbits on dinner-party diplomacy. The cooks insist haute cuisine plays a crucial role in warming ties and sealing international deals. The club, whose title plays on the double meaning of the French word “chef” for cook and leader, was founded 35 years ago...