Duff Goldman got the idea for using Kahlua as an all-purpose flavor enhancer from hotel pastry chefs. He said that it works particularly well in creamy desserts: “You’re making a crème brulee, put [Kahlua] in there. You make some buttercream, put it in there. Literally anything — a chocolate pot du crème, put some Kahlua in there, just a little bit.”
As for why Kahlua makes everything better, Goldman chalks it up to the mysterious mix of flavorings in the liqueur. Although Kahlua is just made with coffee beans and rum, the ingredients are blended in a way that makes the finished drink uniquely compelling. It’s fairly easy to make a Kahlua substitute at home, but it might not have the same magical properties in pastries as the real thing. In his words, “Whatever the flavor balance that the people who make Kahlua have figured out, it’s a well-designed flavor that tastes good to us. It pushes all the right buttons.” He compared it to savory ingredients that improve pretty much every dish they’re used in; he tends to reach for bacon and corn to rescue savory recipes that taste a bit flat.