Preheat your oven to 350°F.
Let your baked pie crust cool completely before you start with the filling.
In a saucepan, combine the lemon pudding mix, 1/3 cup sugar, and 2 1/4 cups water.
Stir well.
Blend the 3 egg yolks in a bowl.
Add a small amount of the pudding mixture to the yolks and mix, then pour it all back into the saucepan.
Cook the mixture over medium heat, stirring constantly, until it thickens and boils.
Remove from heat.
Stir in lemon juice and butter.
Pour filling into the cooled crust.
In a clean mixing bowl, beat the egg whites with cream of tartar until soft peaks form.
Gradually add 6 tablespoons of sugar and keep beating until stiff peaks form.
Spread the meringue over the hot lemon filling, making sure to seal the edges.
Bake for 12–15 minutes, or until the meringue is golden brown.
Cool at room temperature, then refrigerate for at least 2 hours before slicing.