Melt butter in medium saucepan over medium heat and sweat onions and garlic. Add rum, bring to a boil and reduce to 1/4 cup. Add stock, bring to a boil and reduce heat. Whisk in ancho puree, molasses and pepper and simmer until reduced to 1 cup. Preheat grill. Season steaks on both sides with salt. Cook for 2-3 minutes on each side until medium-rare.