Cook bacon in a skillet until desired crisp-ness. While bacon cooks, toast bread. Spread each of 4 slices of toast with 1/2 Tbsp. butter. Mash avocado; spread mashed avocado evenly over each of the unbuttered pieces of toast. Place lettuce on avocado, top with tomato, 2 slices bacon, and remaining toast. Top with mustard, if desired.