beat egg yolks on high speed in a medium bowl for 2 to 3 minutes or until light yellow. add sweetened condensed milk and beat again for another 30 seconds.
transfer this mixture into a medium saucepan over med/low heat and stir in 1/2 cup key lime juice. continue heating for 8 to 10 minutes, stirring often, until mixture begins to bubble. remove key lime filling from the heat and cool, then cover and chill for 3 to 4 hours.
make the key lime topping by combining sour cream, key lime juice, and powdered sugar. cover and chill.
to build each serving, mix the melted butter with 1/2 cup graham cracker crumbs. spoon 2 tbsp graham cracker crumbs into a 6-ounce rocks glass. spoon in 1/3 cup key lime filling, and top it off with 2 tbsp of the topping. add a pile of whipped cream, sprinkle about 1/2 tsp graham cracker crumbs on the whipped cream and serve.