Preheat oven to 350°.
In large bowl, combine flour, sugar and cocoa; mix well.
Cut in margarine until crumbly (mixture will be dry).
Reserving 2 cups crumb mixture, press remainder firmly on bottom of 13x9-inch baking pan.
Bake 15 minutes.
Meanwhile, in large mixer bowl, beat cheese until fluffy.
Gradually beat in sweetened condensed milk until smooth.
Add egg and vanilla; mix well.
Pour evenly over baked crust.
Combine reserved crumb mixture with nuts if desired; sprinkle evenly over cheese mixture.
Bake 25 minutes or until bubbly.
Cut into bars.
Store covered in refrigerator.