Step 1: Preheat your oven to 350°F (175°C). Lightly grease and flour (or line with parchment paper) a 9×13 inch baking pan to prevent sticking and ensure easy removal.
Step 2: In a large mixing bowl, combine the dry lemon cake mix and the melted unsalted butter. Mix with a fork or your hands until the mixture is thoroughly combined and resembles coarse, wet crumbs.
Step 3: Press about two-thirds of this cake mix mixture evenly and firmly into the bottom of the prepared 9×13 baking pan. This will form the base crust for your lemon bars.
Step 4: Pour the entire can of lemon pie filling (or lemon curd) evenly over the pressed crust, spreading it to the edges with a spatula.
Step 5: Sprinkle the remaining one-third of the cake mix mixture over the lemon filling. This creates a delightful crumbly topping.
Step 6: Bake for 25-30 minutes, or until the topping is lightly golden brown and the lemon filling appears set. The edges may be slightly bubbly.
Step 7: Remove the pan from the oven and let the lemon bars cool completely on a wire rack. This is crucial for the filling to set properly and for clean cuts.
Step 8: Once completely cooled, dust with powdered sugar if desired. Cut into squares and serve. These lemon bars are best enjoyed chilled!