PREPARE brownie batter as directed on package; spread evenly into 2 greased half-sheet pans (or 1 half sheet pan for trial recipe).
PLACE cream cheese, sugar and vanilla in large bowl with electric mixer fitted with paddle attachment. Beat on medium speed until well blended. Add eggs, 2 at a time, mixing just until blended after each addition. Pour over batter in pan(s); cut through batter with knife several times for marble effect. Sprinkle with chocolate chips, if desired.
BAKE in 350°F-convection oven for 30 to 35 minutes or until cream cheese mixture is lightly browned. Cool in pan on wire rack. Cut into 3-inch squares.