01 -
Place bacon in a cold skillet over medium heat. Cook, turning occasionally, until deeply crisp, 6 to 8 minutes. Transfer to paper towels to drain.
02 -
Halve the avocado, remove the pit, and scoop the flesh into a bowl. Mash with lime juice, salt, and cilantro if using. Leave it slightly chunky.
03 -
Spread softened butter on one side of each bread slice. On the unbuttered sides, divide half the cheese among two slices. Top with bacon, then guacamole, then the remaining cheese. Close with the other bread slices, butter facing out.
04 -
Heat a skillet over medium-low heat. Add the sandwiches and press gently with a spatula. Cook for 3 to 4 minutes per side until the bread is deeply golden and the cheese is fully melted.
05 -
Transfer to a cutting board and let rest for 1 minute. Slice in half and serve immediately.