Preheat the oven to 350 degrees F. Spray a 12-cup muffin tin with cooking spray.
Crack the eggs into a large bowl, add the salt and whisk to combine. Divide the mixture evenly among the muffin cups. Top with desired variation ingredients.
Bake until the centers of the muffins no longer jiggle, 17 to 20 minutes.
Let the muffins cool in the pan for 5 minutes. Run a knife around the edges of the muffins, unmold and serve.
If storing, allow to cool completely, place in a resealable plastic bag and refrigerate or freeze.
To reheat, microwave the muffins one at a time for about 30 seconds if refrigerated, 1 minute if frozen. You can also heat them in a 250-degree-F oven until warmed through.
