Preheat the oven to 375°F. Spray an 8-inch or 9-inch square pan with cooking spray and line it with parchment paper, leaving an overhang for easy removal.

In a bowl, combine 1½ cups graham cracker crumbs, ⅓ cup granulated sugar, and 6 tablespoons melted butter. Stir until evenly moistened.

Press the graham cracker mixture firmly and evenly into the bottom of the prepared pan (use the bottom of a measuring cup to pack it tightly). Bake the crust for 7 minutes. Remove from the oven and let the crust cool until it is no longer hot to the touch.

While the crust cools, preheat the oven to 350°F.

In a medium bowl, whisk together 14 ounces sweetened condensed milk, 5 egg yolks, ½ cup lemon juice, and 2 tablespoons lemon zest until smooth and combined.

Pour the lemon filling over the cooled crust and smooth the top gently. Bake for 25 minutes, or until the edges are set and the center is slightly jiggly. Remove from the oven and let cool to room temperature.

After the bars reach room temperature, refrigerate them for at least 2 hours before serving to fully set.

(Optional) To make candied lemon zest: combine 2 tablespoons lemon zest and ¼ cup granulated sugar in a small bowl. Toss until the zest is completely coated, then let sit for 30 minutes. Use this to top the bars before serving.

(Optional) If you are not using the candied lemon zest, you can sprinkle 2 tablespoons key lime and/or lemon zest (fresh) over the bars instead. Dust with powdered sugar if desired.

Lift the chilled slab from the pan using the parchment overhang, cut into bars, and store in an airtight container in the refrigerator.