1. Use a food processor or a potato masher to smash beans until chunky, leaving some partly whole. Stir in crushed chips, salsa, cheddar, egg, scallion and black pepper. Set mixture aside 10 minutes, then shape into 4 patties.
2. Place patties on a plate, cover with plastic wrap and refrigerate at least 30 minutes or overnight.
3. Preheat the oven to 350 degrees. Place buns on a baking sheet and heat in oven until warm to the touch, 4 to 6 minutes.
4. Meanwhile, heat 1 teaspoon oil in a large nonstick skillet over medium heat. Add patties to pan and cook until golden brown on one side, 4 to 5 minutes. Add remaining oil, flip patties and cook until underside is golden, 4 to 5 minutes. Remove from heat and serve topped with salsa and lettuce, if desired, on buns.