Crust:

1. Line an 8×8 baking dish with aluminum foil. Spray lightly with cooking spray.
2. Combine graham cracker crumbs, sugar and melted butter in a medium bowl and mix well. Press graham cracker mixture evenly into the bottom of the prepared pan. Bake crust at 325°F for 15 minutes. Let cool.

Filling:

1. In a medium bowl, mix sweetened condensed milk, key lime juice, sour cream and egg yolk. Mix until well combined. Pour filling into pan with cooled crust.
2. Bake in a shallow water bath at 325°F for 40 minutes.
3. Remove from water bath and let cool for about an hour then cover and chill in the refrigerator for at least an 2 -3 hours, but preferably overnight.
4. When ready to serve, pull foil out of dish and carefully fold it down, releasing the bars.

Topping:

1. In a small bowl, whip the cream, sugar and vanilla until it forms stiff peaks.
2. Top bars with whipped cream and sprinkle with shredded coconut (if desired).