Heat oven to 350 degrees F and line 9x13 pan with foil and cover with bake spray.
To make crust
Place the dry cake mix, butter, 1 egg and lemon zest in a bowl and mix until well blended. Press mixture into parchment-lined 9x13 inch pan. Set aside.
To make berry custard filling
1.Place egg in a bowl of stand mixer fitted with a whisk attachment. Beat eggs until well combined. Add in sugar and beat and until foamy. Add in sour cream, flour, cornstarch and mix to combine. Turn off mixer and fold in raspberries, blueberries and lemon zest.
2.Pour mixture on top of crust and bake at 350 degrees for 40-55 minutes or until the edges are a light golden brown, and the center slightly jiggles when pan is tapped.
3.Cool completely in pan. Turn out onto a cutting board and cut into squares.