Both butter and mayonnaise produce grilled-cheese sandwiches with crisp, well-browned exteriors.
Some cooks we know swear by spreading mayo on the bread for grilled cheese instead of butter. Since it saves the step of having to soften butter to make it spreadable, we decided to test this method ourselves. Both butter and mayonnaise produced sandwiches with crisp, well-browned exteriors. Unsurprisingly, the sandwiches made with butter tasted more buttery, while the mayonnaise versions had a slight but discernible tang. But both samples were deemed entirely acceptable.
A word of warning: Though we’re fans of light mayonnaise (our favorite light mayo closely rivaled our favorite full-fat version in a side-by-side taste test), it won’t work here. Because the low-fat product has only one-third of the fat of regular mayonnaise (water is the first ingredient listed), it makes a pale, soggy grilled cheese.