In a medium pot, combine beans and their liquid, Southwest Spice, 1 TBSP butter, ¼ tsp salt (½ tsp for 4 servings), and pepper. Bring to a boil over medium-high heat. Once boiling, reduce heat to medium. Let simmer, uncovered, for 5 minutes, then mash beans until mostly smooth. Continue simmering until mixture has thickened, 3-5 minutes more. Turn off heat; cover to keep warm.