1Heat oven to 350°F (325°F for dark or nonstick pan). Grease or spray bottom only of 13x9-inch pan. In large bowl, beat cake mix, butter, egg and milk with spoon or electric mixer on low speed until well blended. Spread batter in pan.
2Bake 16 to 18 minutes or until center is set (top may appear dry and cracked). Cool completely, about 1 hour.
3Brush 2 tablespoons liqueur over cake. Let ice cream stand about 15 minutes at room temperature to soften. Spread ice cream over cake. Freeze about 3 hours or until firm.
4In medium bowl, mix frosting and 2 tablespoons liqueur; spread over ice cream. Freeze at least 1 hour. Store covered in freezer.