Preheat the oven to 400F. Line to baking sheets with parchment. Place the bacon in a single layer on one sheet, and place the bread on the other. Melt butter and evenly distribute with a spoon or basting brush evenly on each slice of bread. Put bacon on the top rack and sheet with bread on the bottom rack of the oven and bake for 15-18 minutes until both are crisp, but not burnt.
While bacon and bread bake, make the basil mayonnaise. Add Mayo, basil and olive oil to a food processor and process until well combined - about one minute. Alternative, roughly chop all basil and stir to combine in a bowl with mayonnaise and olive oil.
when bacon and bread are done, remove from the oven. Place bacon onto a paper towel-lined plate to absorb some grease.
To assemble each sandwich, start with spreading a spoonfuls or two of basil mayo onto one side of each piece of toasted bread. Top the bottom piece of toast with a few pieces of lettuce, followed by 3-4 slices of tomato, 3 slices of bacon, and finish with last piece of toasted bread with basil mayo. Slice and enjoy!
*Pro tip: if you have extra fresh basil, add a few leaves to each sandwich - you won't be disappointed!