Preheat oven to 400°F. Line a sheet pan with foil and arrange bacon in a single layer.
Bake bacon until browned and crisp, 12–15 minutes. Transfer to paper towels to drain.
Rinse and dry lettuce. Slice tomato and blot excess moisture with a paper towel.
Toast bread lightly. Spread butter on one side if using, then toast briefly for golden edges.
Spread mayonnaise over one side of each slice of toast.
Layer bacon on two slices. Add tomato and lettuce, then season with salt and pepper.
Top with remaining bread, mayo side down. Press gently, slice, and serve warm.