Step 1: Preheat your oven to 350°F (175°C). Prepare a 9x13 inch baking pan by lining it with parchment paper, leaving an overhang on the sides for easy removal.
Step 2: Measure out 1 ½ cups of the dry yellow cake mix and set it aside in a small bowl. This portion will be used for the crumb topping.
Step 3: Pour the remaining dry cake mix into the bottom of the prepared 9x13 inch baking pan. Press it firmly and evenly across the entire bottom of the pan to form the crust.
Step 4: Carefully spread the entire can of lemon pie filling over the pressed cake mix crust. Use an offset spatula or the back of a spoon to spread it evenly from edge to edge.
Step 5: Sprinkle the reserved 1 ½ cups of dry cake mix evenly over the lemon pie filling layer. This creates the crumbly topping.
Step 6: Place the baking pan in the preheated oven and bake for 30-35 minutes, or until the topping is lightly golden brown and the lemon filling appears set. A toothpick inserted into the cake mix topping should come out clean.
Step 7: Once baked, remove the pan from the oven and let the bars cool completely on a wire rack. For best results and clean slicing, chill the bars in the refrigerator for at least 2 hours, or ideally overnight. Once chilled, use the parchment paper overhang to lift the slab of bars out of the pan and onto a cutting board, then slice into 12 squares or rectangles. For neatest cuts, wipe your knife clean between each slice.