01 - Preheat oven to 350°F. Line an 8-inch square baking pan with parchment paper, leaving overhang for easy removal.
02 - In a medium bowl, combine graham cracker crumbs, sugar, melted butter, and salt. Mix until texture resembles wet sand.
03 - Press crust mixture firmly and evenly into bottom of prepared pan. Bake for 8 minutes. Remove from oven and let cool slightly.
04 - Whisk egg yolks in a large bowl until slightly thickened, about 1 minute. Add sweetened condensed milk and whisk until smooth.
05 - Stir in key lime juice, zest, food coloring if using, and milk. Mix until fully combined and creamy.
06 - Pour filling over cooled crust and spread evenly.
07 - Bake for 14 minutes, or until center is set but still slightly jiggly.
08 - Let cool to room temperature, then refrigerate for at least 2 hours until fully chilled and set.
09 - Lift bars out using parchment overhang. Cut into 16 squares. Garnish with whipped cream and lime zest if desired. Serve chilled.