2 tablespoons (30g) unsalted butter, room temperature
Heat a cast iron griddle or skillet over medium-high heat until very hot, about 2 minutes.
Gently mix together bison and chorizo in large bowl until well combined.
Brush pan with oil. Divide meat into four, sprinkle one side of each portion liberally with salt and place, salt side down onto grill. Smash burgers into approximately 4 to 5-inch (10-12 cm) rough patties with a spatula and season tops of patties with salt. Cook until edges brown, about 3 to 4 minutes. Flip burgers and cook for another minute before topping with cheese, if using. Continue to cook until 160F (71C), about 2 min. Remove burgers to a plate and rest for 3 minutes.
Turn heat down to medium. Butter cut sides of buns and place butter-side down on griddle. Toast until golden, about 2 min.