01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving an overhang on two sides for easy removal.
02 - In a medium bowl, combine graham cracker crumbs, sugar, melted butter, and salt. Mix until the texture resembles wet sand.
03 - Press the mixture evenly into the bottom of the prepared pan. Bake for 10 minutes, then remove and let cool slightly.
04 - In a separate bowl, whisk together sweetened condensed milk, lime juice, lime zest, and egg yolks until smooth. Add green food coloring if desired and mix until evenly colored.
05 - Pour the filling over the cooled crust. Smooth the top with a spatula.
06 - Bake for 15 minutes, or until the center is set but still slightly jiggly when gently shaken.
07 - Cool to room temperature, then refrigerate for at least 2 hours, or until fully chilled and set.
08 - Whip the heavy cream with powdered sugar until soft peaks form. Spread or pipe over the chilled bars.
09 - Garnish with key lime slices or extra zest. Slice into 16 bars and serve chilled.