Preheat the oven to 350 degrees F (180 degrees C). Lightly coat a 13- x 9-inch baking dish with cooking spray; set aside.
Combine cake mix, eggs, water, and vegetable oil in a large bowl. Beat with an electric mixer until combined; fold in 1 tablespoon lemon zest. Pour batter into the prepared baking dish, spreading it into an even layer.
Bake in the preheated oven until a wooden pick inserted into the center comes out clean, 22 to 26 minutes. Transfer to a wire rack and let cool in the baking dish, about 45 minutes.
Microwave 5 tablespoons of the cream in a large microwave-safe bowl on HIGH until heated, 30 to 45 seconds. Add 1 cup of white chocolate; do not stir. Let stand, undisturbed, for 5 minutes. Stir gently until chocolate is melted and mixture is smooth, about 45 seconds. Pour white ganache in an even layer over the cooled cake. Do not wipe bowl clean.
Add remaining 1 1/2 cups white chocolate to the same bowl and microwave, stirring every 30 seconds, until melted and smooth, 1 to 1 1/2 minutes. Measure 2 tablespoons melted white chocolate and set aside for garnish.
Add lemon curd, salt, remaining 1/2 tablespoon lemon zest, and 2 tablespoons cream; stir to combine. Add cornflakes and stir until evenly coated. Spoon cornflake mixture evenly over cake, gently spreading with the back of the spoon into an even layer. Drizzle with reserved white chocolate. Serve immediately while topping is still runny. Recipe developed by Melissa Gray-Streett
