Preheat oven to 350.  Line a 13x9 inch pan with non-stick foil or parchment. If you prefer not to line the pan, that is fine.

Combine flour, sugar, and cocoa in a large bowl and then cut in the butter until crumbly and rather dry. This can also be done in a food processor.

Reserve 2 cups of the mixture for the topping, then press the remaining into the bottom of prepared pan. Bake crust for 15 minutes.

Beat cream cheese until fluffy, then gradually beat in condensed milk until smooth. Add egg and vanilla. Pour cream cheese mixture over crust. If using nuts, combine with the 2 cups of reserved chocolate mixture. Sprinkle over cream cheese mixture. Bake 25 minutes until bubbly. Sprinkle chocolate chips over top as soon as you remove from oven. Chill thoroughly.  Lift from pan by grasping foil or parchment, then cut into squares.

Makes about 32 squares

Note: You can halve the recipe and make it in an 8 inch square pan. Halve everything, use two tablespoons of egg and cut about 2 minutes of each cook-time listed.