In a food processor combine chipotle chili and mayonnaise. Pulse until smooth; set aside.
For the sandwich:
Heat a large skillet over medium-high heat. Add bacon and cook until crispy, about 5 to 7 minutes. Remove from pan and set aside on paper towel-lined plate.
Heat a clean pan over medium heat. Butter one side of each slice of bread. Spread a layer of chipotle mayo over each slice of bread. Layer the lettuce, onion, tomato, cheese and guacamole over 2 slices of bread, dividing ingredients evenly. Top with remaining bread slices.
Grill over medium heat until golden brown and the cheese has melted, about 2 to 3 minutes per side.