Pre-heat oven to 350F (conventional) and prepare a 9x9" square baking pan by lining it with 2 perpendicular sheets of parchment paper. Make sure the parchment paper is rising to the top of the pan as that will allow you to remove the bars from the pan once they have set

Grind the graham crackers into a find crumb in your food processor. Mix with the melted butter

Transfer the crumbs to the prepared pan and press down tightly to form a crust on the bottom, using the back of a spoon and / or measuring cup

Bake the crust for ~8-10 minutes, then reduce the oven temperature to 325F

In the meanwhile, zest and juice the key limes

Mix the zest, key lime juice, sweetened condensed milk, vanilla and sour cream (or yogurt) with a hand whisk until combined. I recommend starting with ½ cup lime juice and 1 ½ cans of sweetened condensed milk, tasting and adjusting from there

Add the egg yolks and whisk to combine

Transfer the filling to the pan on top of the baked crust. Shake and tap the pan on the counter to get rid of any air bubbles

Bake for ~17-18 minutes at 325F until the filling looks somewhat set around the edges but gently jiggles in the middle when the pan is shaken. It will not be fully set, as it will set further as it chills

Refrigerate overnight, or at least for 4 hours

When the bars are set, prepare the whipped cream topping by whisking the heavy cream with sugar until it reaches stiff peaks. Do not overmix

Slather the whipped cream on top of the filling inside the pan

Carefully remove the bars from the pan by lifting the parchment papers and transfer to your serving board

Cut into squares with a sharp knife, serve and enjoy!