Preheat oven to 350 degrees F. Butter a 9x9x2-inch metal baking pan. Place brownie mix in bowl. Add oil, eggs and liqueur; whisk until blended. Stir in chocolate chips and nuts. Transfer batter to prepared pan. Bake until slightly puffed and tester inserted into center comes out with some moist crumbs still attached, about 35 minutes. Cool brownies in pan on rack.
Makes about 16 brownies.