Preheat oven to 350°F (175°C) and grease an 8×8-inch baking dish.

In a medium bowl, mix flour, powdered sugar, and softened butter until crumbly.

Press the mixture firmly into the bottom of the prepared dish.

Bake for 15 minutes or until the edges are very lightly golden.

In a saucepan, combine granulated sugar, cornstarch, and water.

Cook over medium heat, stirring constantly, until the mixture thickens and becomes translucent.

Slowly stir in beaten egg yolks, lemon juice, and lemon zest.

Cook for an additional 2 minutes, whisking constantly to ensure the yolks don’t scramble.

Pour the lemon filling over the warm crust and bake for 10 minutes.

While the filling bakes, beat egg whites and cream of tartar in a clean bowl until soft peaks form.

Gradually add a few tablespoons of sugar (optional) and beat until stiff, glossy peaks form.

Spread the meringue over the hot lemon filling, sealing the edges to the sides of the pan.

Bake for 10-15 minutes until the meringue is golden brown.

Allow to cool completely at room temperature before chilling and slicing.