Line a muffin pan with cupcake liners and set aside.
In a bowl, mix graham cracker crumbs and sugar. Add melted butter and stir until evenly combined and crumbly.
Divide the crust mixture into each liner and press down firmly to form a solid base.
Chill the crusts in the fridge for 10 to 15 minutes to set.
In another bowl, mix sweetened condensed milk, key lime juice, and lime zest until smooth and slightly thickened.
Spoon the filling evenly into each liner, filling almost to the top.
Bake at 350°F (175°C) for 10 to 12 minutes, until centers are set but still soft.
Remove from oven and let cool completely at room temperature.
Refrigerate for at least 2 hours until fully firm.
Top each bite with whipped topping before serving.