Preheat the oven to 325 degrees F (165 degrees C).

Mix together flour, butter, and 2 tablespoons sugar in a medium-sized bowl. Pat crust into a 8-inch square pan, bringing up the sides.

Bake in the preheated oven until golden brown, about 10 minutes. Let cool completely.

While crust is cooling, make a slurry of cornstarch and water in a small bowl. Set aside. Beat separated egg yolks in a small bowl. Pour yolks into a large saucepan; add rhubarb and 1 1/2 cups sugar, adjusting to tartness of rhubarb. Bring to a boil over medium heat, about 5 minutes. Add cornstarch slurry to boiling rhubarb as needed to thicken mixture. Pour into the cooled crust.

Beat egg whites, 3 tablespoons sugar, vanilla extract, and salt together in a separate bowl until stiff peaks form. Spread completely over rhubarb filling.

Bake in the preheated oven until top is golden brown, about 10 minutes.