• Preheat oven to 350°F.

Make the crust:

• 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.