Line a plate with paper towels. In a large nonstick skillet, cook the bacon over medium heat until very crisp. Drain the bacon on the paper towels. Remove the bacon fat from the pan and wipe the pan with a paper towel, but do not wash it. Set aside.
To make the tortilla chip butter, put the chips in the bowl of a food processor and process until the texture is very fine, similar to sand. Alternatively, place the chips in a sturdy plastic bag. Using a meat mallet or other heavy object, pound the chips until they are the texture of sand.
Put the butter in a medium bowl and add the ground chips. Using a fork, work the chip “sand” and butter together until well mixed. The mixture will be somewhat stiff.
To assemble: Spread the butter mixture on one side of each slice of bread. Place 4 slices, butter-chip mixture side down, on your work surface. Spread 2 tablespoons of the guacamole on each slice of bread. Sprinkle the tomato on top of the guacamole, if using. Follow with the Colby and Monterey Jack Cheeses. Dot with small pieces of the goat cheese. Finish by placing two bacon pieces on each sandwich. Top with remaining bread slices, buttered side up.
For stovetop method: Heat a large nonstick skillet over medium heat for 2 minutes. Put the sandwiches into the pan, cover, and cook for 3 to 4 minutes, until the undersides are golden brown. Watch carefully because the chips in the butter can burn easily. Turn the sandwiches, pressing each one firmly with a spatula to compress the filling slightly. Cover and cook for 2 to 3 minutes or until the undersides are well browned. Turn the sandwiches once more, press firmly with the spatula again, cook for 1 minute, and remove from the pan. Let cool 5 minutes. Cut in half and serve.
For sandwich maker method: Use your sandwich maker for this sandwich only if you have variable heat settings. Otherwise, it will cook too hot and burn the chips on the bread without melting the cheese. To use your sandwich maker, follow directions for assembly above. Cook according to manufacturer’s instructions.
Note: There’s no need to use the tomato if your guacamole already has tomato in it. Guacamole Split and pit the avocado, and mash the avocado flesh in a medium bowl with the lime juice and salt, using a potato masher or a fork. Do not use a blender or food processor. You want to keep the avocado slightly chunky, not make it soupy. Stir in the onion, cilantro, chile, and tomato. Let sit for about 15 minutes to allow the flavors to meld.
Note: This makes more than you need for the sandwiches. Serve the remaining guacamole with chips alongside.