Prepare brownie mix to package directions and spread into well greased 13 X 9 pan.
In a bowl beat cream cheese, margarine and corn starch until fluffy. Gradually beat in sweetened condensed milk, then egg and vanilla until smooth.
Pour evenly over brownie batter.
Bake about 40-45 minutes until lightly browned.
Cool completely. Then ice with frosting desired. I don't use frosting, I melt milk chocolate chips with a touch of shortening in a zip lock bag in microwave snip corner off and drizzle over top of brownies.