In a bowl of a stand mixer, whisk together the egg yolks and sweetened condensed milk until combined. Add in the key lime juice and zest and continue to beat for another 3 minutes.
Pour the filling into the prepared pie crust. Place the pie on a baking sheet and bake for 20 minutes.
Let the pie cool to room temperature, and then chill for 3 hours or overnight in the fridge.
When you're ready to serve, beat the heavy cream until soft peaks form. Then add in the powdered sugar and beat to combine.
Spread the whipped cream over the pie in an even layer. Sprinkle with lime zest and garnish with lime slices if desired. Cut into 8 slices and enjoy.
Notes
You will need 16-20 key limes for 1/2 cup of juice. You can substitute a 1/4 cup of lemon juice + 1/4 cup of lime juice if you can’t get key limes.