Preheat the oven to 350°F and lightly grease a clear 9x13-inch glass baking dish with butter or nonstick spray.

Pour the dry lemon cake mix into a medium mixing bowl, add the melted and slightly cooled butter, and stir with a fork or spatula until all the cake mix is moistened and soft, damp crumbs form with no dry patches.

Press about two-thirds of the buttery cake mixture evenly and fairly firmly into the bottom of the prepared baking dish, reaching into the corners.

Spoon the entire can of lemon pie filling over the crust and gently spread it from edge to edge without digging into the crust.

Sprinkle the remaining one-third of the cake-and-butter crumbs evenly over the lemon filling without pressing them down.

Place the dish on the center oven rack and bake for 25 to 30 minutes, until the topping is lightly golden in places and the lemon filling bubbles gently around the edges.

Remove the pan from the oven and let the bars cool completely in the glass dish on a cooling rack so the lemon layer can set.

Refrigerate the cooled bars for at least 1 to 2 hours, then use a sharp knife to cut them into 16 squares, wiping the knife between cuts for cleaner edges.

Serve the bars chilled or let them stand at room temperature for a few minutes before serving.