Step 1

PreparationMake the crust: Place rack in the center of oven and preheat to 350°F.

Step 2

Pulse graham crackers, sugar, cinnamon, and salt in a food processor until fine crumbs form. With the motor running, drizzle in melted butter and process until well-combined. Transfer mixture to pie pan and press into bottom and up sides with a flat-bottomed measuring cup.

Step 3

Bake crust until golden brown and set, 8–10 minutes. Transfer to a wire rack and let cool to room temperature, about 15 minutes.

Step 4

Make the filling and bake pie: Whisk condensed milk and yolks together in a large bowl. Add key lime and lemon juice and whisk until well-combined (mixture will thicken slightly).

Step 5

Pour filling into crust and bake until center is just set, about 15 minutes. Transfer pie to a wire rack and let cool completely (filling will set as it cools), about 1 hour. Transfer to refrigerator and chill until cold, at least 1 hour.

Step 6

Make the topping: When ready to serve, whip cream with a whisk or electric mixer to very soft peaks. Sprinkle sugar over cream and continue whipping until it holds stiff peaks. Dollop cream in center of pie (leaving at least a 1" border of visible pie filling) and serve.

Step 7

Do Ahead Pie (without topping) can be made, loosely covered, and chilled for up to 2 days.