In a large pot, melt half of the butter on medium heat. Add the onions and sweat until the onions are translucent, 3 minutes. Add the chili powder, garlic salt and paprika, and saute for 2 minutes. Add the ketchup, molasses, honey, mustard, vinegar and Worcestershire sauce. Bring the sauce to a boil then reduce to a simmer and cook for 5 to 10 minutes, stirring frequently.
Transfer the sauce to a blender or use an immersion blender and blitz, leaving some texture to the sauce. Fold in the remaining butter.