You know I love a good mayo hack, but cooking a grilled cheese sandwich in mayonnaise instead of butter is not one of them. You can read my full screed against the practice here, but to sum it up:
I want to like it. In theory, it makes sense, and I want to believe that mayo is some sort of miracle condiment that makes everything it touches better. Mayo’s higher smoke point gives the cheese more time to melt (which I have never needed), and it does crisp up nicely, but it just doesn’t taste like a grilled cheese. Melted butter tastes good. Melted mayonnaise tastes like greasy nothing. Give me butter. Give me flavor.
If you prefer mayo because it spreads easier, or if you forgot to set out some butter to soften, I have an elegant solution:
There’s a very easy way to butter your bread with straight-from-the-fridge butter: Just melt it in the pan first.
This is actually how my stepmom butters all her breads, but it works extremely well with a grilled cheese. Take a tablespoon or so of butter, melt it over medium-low heat, then place a piece of bread in the pool of butter and swirl it around to coat. Remove it from the pan, set it aside (butter side up), and add a little more butter to the pan before adding your next piece of bread. Swirl that piece around in melted butter, add your cheese slices, then place the first buttered slice (butter side up) on top of the cheese. Cover the pan for a minute to help any harder cheeses melt (if you have insisted on using them), and fry and flip until both sides are golden brown.