1. Prepare the Crust: First, preheat your oven to 375°F (190°C). Lightly spray an 8-inch or 9-inch square baking pan with cooking spray, then line it with parchment paper, leaving an overhang to easily lift the bars later.
In a small bowl, mix together the graham cracker crumbs, melted butter, and granulated sugar until well combined. This is your simple crust!
Press this crumb mixture firmly into the bottom of your prepared pan. A flat-bottomed measuring cup works great for really packing it in.
Bake the crust for just 7 minutes until it's lightly golden. Take it out of the oven and let it cool completely while you make the filling.
2. Make the Zesty Filling: Reduce your oven temperature to 350°F (175°C). In a medium bowl, whisk together the sweetened condensed milk, egg yolks, fresh key lime juice, and key lime zest until smooth and perfectly combined.
Pour this vibrant lime filling evenly over your cooled graham cracker crust.
Bake for 25 minutes, or until the filling is set and no longer jiggly in the center.
Let the bars cool down completely at room temperature, then transfer them to the refrigerator to chill for at least 2-3 hours (or even better, overnight) before serving. Chilling helps them firm up beautifully!
3. Finish and Serve: Once thoroughly chilled, use the parchment paper overhang to lift the entire slab of bars out of the pan. Dust with powdered sugar and sprinkle with extra key lime or lemon zest if you like a little extra pop of color and flavor. Slice into neat bars and enjoy this refreshing, tangy treat!