Pour sweetened condensed milk and heavy whipping cream into the mixing bowl that you use with your standing mixer.
Using the whisk attachment for your standing mixer, start mixing on a low speed and gradually increase to high speed. Beat for about 5 minutes, until medium-stiff peaks form.
Add lemonade powder and lemon zest and beat on high for 2 minutes.
Add lemon juice and beat on high for 30 seconds, until just combined.
Evenly pour mixture into prepared graham cracker crust, then cover and place in freezer overnight.
Remove pie from the freezer a few minutes before serving to making cutting slices easier. If desired, serve with whipped topping.
