Spread 1 side of each tortilla with 1 1/2 tsp. softened butter.
Place tortillas, buttered-side down, onto large cutting board. Place 2 slices cheese onto half of each tortilla. Top evenly w/bacon, tomato, & avocado. Fold each tortilla in half over filling.
Heat 12 in. skillet over med. heat; place 2 quesadillas into skillet. Cook, turning once, until golden brown on each side & cheese is melted (5-6 min). Repeat w/remaining quesadillas. Cut each quesadilla into 4 (4 wedge) servings.