Make the filling: In a large mixing bowl, whisk the condensed milk and egg yolks until smooth and thick. Add the key lime zest and juice and whisk for 1 minute.
Assemble and bake the pie: Preheat the oven to 350°F. Place the cooled graham cracker crust on a baking sheet and pour in the filling. Bake on the center rack for 15 minutes, until the edges are set but the center still jiggles like a soft thigh. Let the pie cool at room temperature for an hour before chilling for 3 hours to overnight in the fridge.
Make the topping: Whip the cream to stiff peaks with a stand mixer with the whisk attachment or a hand mixer, 8 to 9 minutes. Top the chilled pie with whipped cream and arrange sliced key limes on top as desired.