If starting from graham cracker sheets, place them in a bag and seal the bag except for the edge (to allow air to escape). Roll with a rolling pin to crush the graham crackers.
Add crushed grahams to the melted butter with the brown sugar and stir with a fork. Press into the bottom and up the sides of a 9-inch pie plate.
Bake the pie crust for 7 minutes and leave the oven on.
Make the filling:
When crust is done pre-baking, whisk together the sweetened condensed milk, egg yolks, key lime juice, and zest. Pour into crust.
Bake an additional 20-22 minutes or until top is set.
Cool completely before topping with whipped cream or serving.
Make the whipped cream:
Beat cold heavy whipping cream with powdered sugar and vanilla on high speed until stiff peaks form. If desired, add cream of tartar during mixing to create a more stabilized cream (see note).
Store pie in refrigerator for up to 3 days. It’s best to top with whipped cream right before serving. Garnish with lime slices, if desired.
You don’t have to add cream of tartar to your whipped cream, but if you are planning to let the pie sit out for a bit or want to top it a few hours before serving, it will help stabilize the whipped cream.
Can’t find key limes? Use bottled Nellie & Joe’s Famous Key West Lime Juice instead and the zest of regular limes.