Preheat and Prepare: First, preheat your oven to 180°C (160°C fan). Lightly grease a 23x33 cm (9x13 inch) baking dish with a little butter.

Create the Lemon Base: Spoon the lemon pie filling or lemon curd into the prepared dish and spread it out into an even layer with the back of a spoon. If you're using it, sprinkle the fresh lemon zest evenly over the top.

Add the Cake Mix: Open the box of cake mix and sprinkle it as evenly as you can over the entire surface of the lemon filling. Do not mix it in! Just gently pat it down to create a level layer.

Add Optional Extras: If you're using them, scatter the white chocolate chips over the surface of the dry cake mix.

Layer the Butter: Now, take your thinly sliced cold butter and arrange the pieces in a single layer over the entire surface of the cake mix. Try to cover as much of the powder as possible. I find that what works best for me is using a cheese slicer to get uniformly thin pieces of butter.

Bake to Perfection: Place the dish in the preheated oven and bake for 45-50 minutes. You'll know it's ready when the topping is a deep golden brown and you can see the lemon filling bubbling up around the edges.

Rest and Serve: Carefully remove the dish from the oven and let it stand for at least 15 minutes before serving. This allows the filling to set slightly, making it easier to scoop. It's absolutely delicious served warm.