Preheat oven to 350 °F (175 °C). In a bowl, combine graham crumbs, melted butter, and ¼ cup sugar. Press into a 9‑inch pie plate evenly across base and up the sides. Bake for 8 minutes until fragrant; let cool slightly.

In a mixing bowl, whisk egg yolks and lime zest until pale. Add condensed milk and whisk until smooth. Stir in key lime juice until well blended—the acid thickens the custard naturally

Pour filling into warm crust and bake 10–12 minutes, until edges are set but center still jiggles slightly. Let pie cool on rack, then chill in the fridge at least 3 hours (best overnight).

For whipped cream topping: whip heavy cream with powdered sugar until soft peaks form; spread over chilled pie.
For meringue: in a clean bowl, beat egg whites with cream of tartar until foamy, then gradually add sugar until stiff peaks form. Spread meringue over cooled pie and bake at 325 °F (160 °C) for 10–15 minutes until golden.

Slice and serve cold for best texture and flavor.