Line an 8x8 baking dish with aluminum foil. Spray lightly with cooking spray.
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.
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.
Bake in a shallow water bath at 325°F for 40 minutes.
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.
When ready to serve, pull foil out of dish and carefully fold it down, releasing the bars.
In a small bowl, whip the cream, sugar and vanilla until it forms stiff peaks.
Top bars with whipped cream and sprinkle with shredded coconut (if desired).
Key limes are not always easy to find, they're small and are kind of a pain to juice, so I just use regular limes in thees bars.