Step 1: Prepare the crust. In a medium bowl, combine the graham cracker crumbs and granulated sugar. Pour in the melted unsalted butter and mix well until the crumbs are evenly moistened.
Step 2: Form the crusts. Line a 12-cup muffin tin with paper liners. Distribute the graham cracker mixture evenly among the liners, pressing it firmly into the bottom of each to form a crust. You can use the back of a spoon or a small glass for this. Place the muffin tin in the refrigerator to chill for at least 15 minutes while you prepare the filling.
Step 3: Make the Key lime filling. In a separate bowl, whisk together the sweetened condensed milk, Key lime juice, and lime zest until smooth and well combined. The mixture will thicken slightly as you whisk.
Step 4: Assemble the pies. Pour the Key lime filling evenly over the chilled graham cracker crusts in the muffin tin liners.
Step 5: Chill until set. Return the muffin tin to the refrigerator and chill for at least 2 hours, or until the filling is fully set and firm.
Step 6: Serve and garnish. Once set, carefully remove the mini pies from the muffin tin. Garnish each pie with a dollop of whipped cream and a thin slice of Key lime or a sprinkle of lime zest, if desired. Serve immediately and enjoy!