In a mixing bowl, combine flour, sugar, and salt. Add the cold butter and mix until the mixture resembles coarse crumbs.
Add the egg yolk and cold water, one tablespoon at a time, until the dough comes together. Press the dough into a tart pan.
Bake the crust for 15 minutes or until golden brown. Let it cool.
For the filling, whisk together sugar and cornstarch in a saucepan. Gradually add orange juice, lemon juice, and lemon zest. Cook over medium heat until thickened.
Remove from heat and whisk in egg yolks and butter until smooth. Pour the filling into the cooled crust.
Chill the tart in the refrigerator for at least 2 hours before serving.