Step 1: Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the long sides for easy removal.
Step 2: In a large bowl, whisk together the flour, 1/2 cup granulated sugar, and salt for the crust.
Step 3: Cut in the cold, cubed butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs. Press this mixture evenly into the bottom of the prepared pan.
Step 4: Bake the crust for 15-20 minutes, or until lightly golden brown. Remove from the oven and set aside.
Step 5: While the crust is baking, prepare the filling. In a medium bowl, whisk together 1 1/2 cups granulated sugar and cornstarch until well combined.
Step 6: Gradually whisk in the fresh lemon juice, fresh lime juice, eggs, lemon zest, and lime zest until smooth.
Step 7: Pour the filling mixture evenly over the warm, pre-baked crust.
Step 8: Return the pan to the oven and bake for an additional 20-25 minutes, or until the center is set and no longer jiggles when gently shaken.
Step 9: Remove the bars from the oven and let them cool completely on a wire rack at room temperature. This can take 1-2 hours.
Step 10: Once completely cooled, arrange the assorted sour candies decoratively over the top of the lemon-lime filling. You can chop larger candies for easier eating and better distribution.
Step 11: Refrigerate the bars for at least 2-3 hours, or preferably overnight, to allow the filling to firm up completely and the flavors to meld. This also makes them much easier to cut.
Step 12: Lift the bars out of the pan using the parchment paper overhang. Slice into 12 squares and serve chilled. Store any leftover bars in an airtight container in the refrigerator.