Combine graham cracker crumbs, sugar, and melted butter in a bowl. Mix until the crumbs are evenly coated and resemble damp sand.
Press the crumb mixture firmly into the bottom and up the sides of an 8-inch pie pan, creating an even crust. Use the back of a spoon or your fingers to smooth it out.
Bake the crust at 350°F (175°C) for about 8 minutes until lightly golden. Remove from oven and let cool completely, about 15 minutes.
While the crust cools, juice the key limes until you have about 3/4 cup of fresh lime juice. Zest two of the limes and set the zest aside for later.
In a mixing bowl, whisk the egg yolks until smooth and slightly pale. Add the sweetened condensed milk, lime juice, and lime zest, stirring until fully combined and smooth.
Pour the lime filling into the cooled crust, spreading it evenly with a spatula. Bake at 350°F (175°C) for about 10 minutes until the filling is just set but still slightly wobbly in the center.
Remove the pie from the oven and let it cool to room temperature. Then refrigerate for at least 4 hours or overnight to let the filling set completely.
In a chilled bowl, whip the heavy cream with sugar until soft peaks form—this should take about 2-3 minutes and produce a light, fluffy texture.
Spread or pipe the whipped cream generously over the chilled pie. Garnish with additional lime zest if desired for extra aroma and color.
Slice the pie with a sharp knife, revealing the vibrant, creamy filling and crisp crust. Serve chilled for a refreshing, tangy treat.
