Preheat the oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper or foil, leaving an overhang on the sides for easy removal. Lightly spray with cooking spray and set aside.

In a mixing bowl, combine the graham cracker crumbs, granulated sugar, and melted butter until fully incorporated. Firmly press the mixture into the bottom of the prepared pan in an even layer. Bake for 10 minutes. Remove from the oven, reduce the oven temperature to 300°F (150°C), and allow the crust to cool slightly.

In a large bowl, beat the softened cream cheese using an electric mixer at medium speed until smooth and free of lumps. Scrape down the sides of the bowl as needed. Add the egg yolks and continue beating until combined. Gradually mix in the sweetened condensed milk, key lime juice, and lime zest until the filling is fully blended and smooth.

Pour the filling over the cooled crust, spreading it evenly. Bake for 35–40 minutes, or until the center is set but slightly jiggly. Remove from the oven and allow to cool at room temperature for 30 minutes. Refrigerate for at least 2–3 hours or until fully chilled and set.

In a separate bowl, whip the cold heavy cream and powdered sugar on low speed until the mixture begins to thicken. Increase to medium-high speed and continue whipping until stiff peaks form.

Spread the whipped cream evenly over the chilled bars. Garnish with additional lime zest. Use the parchment paper overhang to lift the bars from the pan. Slice into squares with a sharp knife, wiping between cuts for clean edges. Serve chilled.