01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a medium bowl, combine graham cracker crumbs, sugar, melted butter, and salt. Press mixture firmly into the bottom of the prepared pan.
03 - Bake crust for 8-10 minutes, then remove from oven and let cool slightly.
04 - In a large bowl, whisk egg yolks until slightly thickened. Add sweetened condensed milk, lime juice, lime zest, and food coloring if using. Whisk until smooth and fully combined.
05 - Pour filling over the cooled crust and smooth the top. Bake for 10-12 minutes, or until set but still slightly jiggly in the center.
06 - Let cool to room temperature, then refrigerate for at least 2 hours or until fully chilled.
07 - Whip the heavy cream with powdered sugar until soft peaks form.
08 - Lift bars from the pan using the parchment overhang. Slice into squares. Top each bar with whipped cream and garnish with lime slices or zest if desired.