Cook the onions in butter in a small saucepan over medium-low heat until tender, about 10 minutes. Stir in the salt and chile powder. Add the beer and simmer for 5 minutes. Remove from heat and cool thoroughly. Place the roasts in a large plastic bag and add the cooled marinade, turning to coat. Close the bag securely and marinate in the refrigerator for 6 to 8 hours (or overnight) turning occasionally.