Combine graham cracker crumbs, ground walnuts (or extra crumbs), sugar, and melted butter. Mix until the crumbs are fully moistened. Press this mixture firmly into a 9-inch pie dish, covering the bottom and working it up the sides. Chill in the refrigerator or freezer for at least 20-30 minutes to set.
While your crust chills, preheat your oven to 350°F (175°C). In a medium bowl, whisk together the egg yolks, sour cream, and sweetened condensed milk until the mixture is smooth. Stir in the fresh lime juice and lime zest until thoroughly combined. Pour this vibrant, creamy filling into your prepared, chilled crust.
Carefully place the pie in the preheated oven and bake for 25-30 minutes. The filling should look set with a slight jiggle in the center, and the crust should turn lightly golden, but ensure the filling itself does not brown. Remove from the oven and let it cool completely on a wire rack. Once cooled, transfer the pie to the refrigerator and chill for at least 3-4 hours, or ideally overnight, until it's perfectly firm. This extended chill time is crucial for the best texture!
When your pie is thoroughly chilled, pour the cold heavy cream into a large bowl. Add the sugar and vanilla extract, then beat with an electric mixer until stiff peaks form. Spread or pipe this fluffy, light topping generously over your chilled pie.
For a beautiful presentation, sprinkle a little extra fresh lime zest and arrange a few thin lime slices on top. Slice and serve this delightful, cool, and healthy-ish treat straight from the fridge!