Preheat the oven to 350* and grease a 9×13 baking pan
Combine the cake mix, pudding, oil, water, eggs, vanilla and lemon zest and mix together, just until combined
Spread evenly in the prepared pan and bake for 30 to 35 minutes, test with a toothpick
Set on wire rack to cool slightly
In a mixing bowl, combine the powdered sugar, lemon zest, half of the lemon juice, whipping cream and butter. Add more of the lemon juice until desired thickness
