Preheat the oven to 350 degrees. Line an 8 inch pie dish with baking paper, ensuring the paper extends over the sides of the dish.
Do not follow the directions on the buttercake package. Instead, melt the butter and add it to the dry buttercake mix, combining well. Press the mixture into the prepared dish. Place in oven & cook 15 minutes, or until lightly browned. Remove from oven, but leave the oven on. Cool crust slightly.
Beat the sour cream, eggs and lemon zest together until smooth, then pour mixture over the apples. Bake for 30 minutes or until topping is set. Sprinkle the cinnamon over the top. Allow to cool to room temperature in dish, then refrigerate until cold.
HINT: This recipe can also be made in a 8x12x1 inch deep sheet tray. Cut into squares this dessert can easily be transported for a picnic.