In a medium bowl, combine avocados, lime juice, sea salt, onion, jalapeño, cilantro, tomato and garlic. Stir until well combined. If making a few hours ahead, place plastic wrap directly on top of the guacamole to prevent browning.
Lay four slices of bread on a cutting board and top with cheese and bacon. Spread the other four slices with a thick layer of guacamole and place face down on the other slices of bread to make four sandwiches.
Heat a skillet to medium-low heat. Melt butter and add sandwiches. Cook until golden brown, about 4 to 6 minutes per side.