Line an 20 cm square pan with foil. Melt butter in a glass bowl on HIGH, 1 minute. Stir in sugar and milk. Cook on HIGH for 8 minutes, stirring every 3 minutes. Add marshmallows and chocolate; stir and reheat until smooth. Stir in nuts and liqueur. Pour into prepared pan. Chill until firm. Cut into pieces. Makes 2 pounds.