Spread about 1 teaspoon of mayonnaise on 1 side of each piece of bread. Spray a large skillet with nonstick cooking spray and heat over medium heat. In the skillet, place as many slices of bread that will fit, mayo side down, and griddle them up until the mayo side of the bread is nicely browned, about 1 minute. Remove the bread from the skillet, let rest browned side up, and griddle up any remaining slices.