Preheat oven to 350 degrees F. Line a 9-inch by 13-inch baking pan with foil (or grease and flour it).
FOR THE BASE:.
Beat the 4 ounces of cream cheese, 1 egg and butter together on low speed until creamed. Blend in cake mix by hand until combined. It will be very thick, but piable like a soft dough; Press into bottom of the prepared baking pan.
FOR THE TOPPING:.
In a bowl, beat together the two 8-oz packages cream cheese, banana, powdered sugar, and vanilla. Beat at medium speed until smooth, at least 3 minutes. Add eggs one at a time, beating only until incorporated.
Spread cream cheese batter evenly over bottom chocolate layer.
Bake at 350 degrees F. for 45 minutes. Cool completely. Carefully remove from pan, peel off foil, and cut into 24 squares.