Prebake the crust:
Preheat oven to 375°F (190°C). Place the pie shell in the oven and bake for 10–12 minutes until lightly golden. Set aside to cool.
Make the rhubarb filling:
In a medium saucepan, combine chopped rhubarb, sugar, cornstarch, and water. Cook over medium heat, stirring constantly, until the mixture thickens and rhubarb softens (8–10 minutes).
Temper the egg yolks:
In a small bowl, whisk egg yolks. Slowly whisk in a bit of the hot rhubarb mixture to temper. Return yolks to saucepan and cook for 2 more minutes. Remove from heat and stir in butter and vanilla. Pour the hot filling into the baked pie crust.
Make the meringue:
In a clean mixing bowl, beat egg whites and cream of tartar until soft peaks form. Gradually add sugar, beating until stiff, glossy peaks form.
Top and bake:
Spread meringue over the hot filling, sealing it to the crust edges. Use the back of a spoon to create peaks. Bake at 350°F (175°C) for 10–12 minutes, or until golden brown.
Cool and serve:
Let the pie cool completely at room temperature before slicing to set the filling.