Step 1: Preheat your oven to 350°F (175°C).
Step 2: In a medium bowl, combine 1 ½ cups graham cracker crumbs, ¼ cup granulated sugar, and ½ teaspoon ground cinnamon (if using). Mix well.
Step 3: Pour in 6 tablespoons of melted unsalted butter. Mix with a fork until the crumbs are evenly moistened and resemble wet sand.
Step 4: Press the mixture firmly into the bottom and up the sides of a 9-inch pie plate. Use the bottom of a glass or a measuring cup to get an even, compact layer.
Step 5: Bake for 8-10 minutes, or until lightly golden brown. Remove from oven and let cool completely on a wire rack.
Step 6: In a large mixing bowl, combine one (14-ounce) can of sweetened condensed milk, ½ cup of fresh Key lime juice, and 1 teaspoon of Key lime zest.
Step 7: Whisk the ingredients together vigorously until well combined and slightly thickened. Do not overmix; just combine until smooth.
Step 8: Pour the Key lime filling into the cooled graham cracker crust. Spread evenly with a spatula.
Step 9: Place the pie in the refrigerator and chill for at least 4 hours, or preferably overnight, until firm.
Step 10: Before serving, garnish with freshly whipped cream and extra Key lime zest or thin lime slices, if desired.
Step 11: Slice and serve chilled. For clean slices, use a warm, sharp knife, wiping it clean between cuts.