Preheat the oven to 375°F (190°C). In a mixing bowl, combine flour and sugar. Add cold butter and mix until crumbly.

Stir in egg yolk and enough ice water to form a dough. Roll out and fit into a 9-inch pie dish. Prick the bottom with a fork.

Bake for 15 minutes or until lightly golden. Remove from oven and let cool.

In a saucepan, combine grapefruit juice, sugar, cornstarch, and salt. Cook over medium heat, stirring until thickened.

Remove from heat and whisk in egg yolks and butter until smooth. Pour filling into the cooled pie crust.

In a clean mixing bowl, beat egg whites and cream of tartar until soft peaks form.

Gradually add sugar, continuing to beat until stiff peaks form.

Spread meringue over the filling, sealing the edges. Create peaks with a spatula.

Bake in the preheated oven for 10-15 minutes, or until meringue is golden brown.

Remove from oven and let cool before serving.