Open the can of lemon pie filling and spoon it over the crust. Gently spread the filling into an even layer with an offset spatula or the back of a spoon, taking care not to pull up the crust underneath. You want a bright, smooth yellow layer from edge to edge.
Sprinkle the remaining 1/3 of the cake-and-butter mixture evenly over the lemon pie filling. Let it fall in small clumps and crumbles rather than packing it down; this will create that slightly crumbly, golden top once baked. Place the baking dish on the center rack of the preheated oven.
Bake for 25 to 30 minutes, or until the top is lightly golden in spots and the lemon filling is bubbling gently around the edges. The center will still look soft but should no longer appear very loose or wet. Remove the dish from the oven and place it on a cooling rack.
