Crumble the bacon and add to a cast iron skillet, set to medium high heat. Once it begins to sizzle, add a tablespoon of unsalted butter and add the eggs.
Scramble until softly set, remove from pan. Add a small pat of butter to pan, place a slice of sourdough bread and a slice of cheddar.
Scoop out a fourth of the bacon and egg mixture and spread evenly across the bread. Top with a slice of mozzarella and the other slice of bread. Flip once the bread is lightly browned and cook on the other side until the bread is golden and cheese is melted.
Repeat with the other slices of bread and serve with slices of cooked bacon.
Note: You can make these ahead, then wrap individually in parchment paper and freeze in a ziplock bag. When you're ready to eat them, pop them in the microwave for 1:30 minutes or until cheese is melted)