1. Line rimmed baking sheet with triple layer of paper towels, spread beans over towels, and let sit for 15 minutes.
2. Whisk reserved bean liquid and flour in large bowl until well combined and smooth. Stir in scallions; cilantro; garlic; cumin; hot sauce, if using; coriander; salt; and pepper until well combined. Process tortilla chips in food processor until finely ground, about 30 seconds. Add black beans and pulse until beans are coarsely ground, about 5 pulses. Transfer bean mixture to bowl with flour mixture and mix until well combined.
3. Adjust oven rack to middle position and heat oven to 200 degrees. Divide mixture into 6 equal portions and pack firmly into 3 ½ inch-wide patties.
4. Heat 1 tablespoon oil in 10-inch nonstick skillet over medium heat until shimmering. Gently lay 3 patties in skillet and cook until crisp and well browned on first side, about 5 minutes. Gently flip patties, add 1 tablespoon oil, and cook until crisp and well browned on second side, 3 to 5 minutes.
5. Transfer burgers to wire rack set in rimmed baking sheet and place in oven to keep warm. Wipe out skillet with paper towels and repeat with remaining 2 tablespoons oil and remaining patties. Transfer to buns and serve.