PREHEAT oven to 350°F. Mix dressing, eggs and water until well blended. Stir in brownie mix just until moistened.
CHOP 2 of the chocolate bars; stir into brownie batter. Pour into greased 13x9-inch baking pan.
BAKE 30 to 35 minutes or until edges begin to pull away from sides of pan. Chop remaining chocolate bar; immediately sprinkle over warm brownie. Let stand 5 minutes or until chocolate is melted; spread evenly over brownie. Top with walnut halves. Cool. Cut into 32 brownies.