Prepare brownie mix as directed on package, adding vanilla. Reserve 1 cup batter. Spread remaining batter in greased 9-inch square baking pan.
Beat cream cheese, flour and sugar in medium bowl with electric mixer on medium speed until smooth. Add Irish cream liqueur, egg and food color; beat until well blended. Pour over brownie layer in pan. Drop reserved 1 cup batter by spoonfuls over cream cheese layer. Cut through batter with knife several times for marble effect.
Bake as directed on package for 9-inch square baking pan. Cool in pan on wire rack. Cut into squares. Serve with a small scoop of vanilla ice cream, if desired.