Bison Burgers With Barbecue Sauce

 Melt 2 tablespoons of the butter in a small saucepan over medium heat. Add garlic and chipotle; cook, stirring often, until garlic begins to turn golden, 1 to 2 minutes. Stir in vinegar, ketchup, honey, and tomato paste; bring to a simmer. Reduce heat to medium-low and cook, stirring occasionally, until thickened, 2 to 3 minutes. Remove from heat.

 Melt remaining 2 tablespoons butter; let cool to room temperature. Combine bison, melted butter, salt, and pepper in a medium bowl; stir gently until just combined. Gently shape mixture into four 4-inch patties.

 Heat a grill pan or grill to medium-high (about 450˚F); lightly coat grill pan or grates with oil. Add patties; cook until a thermometer inserted into thickest portion registers 160˚F, about 4 minutes per side, or to desired degree of doneness. Remove and let stand for 5 minutes.

 Place 1 patty on each lettuce leaf. Top each with 1 1/2 tablespoons of the barbecue sauce, tomato slices, and pickle chips. Wrap leaf around patty to form a “bun.”