Preheat the oven to 350°F (175°C) and grease a 9-inch pie dish with butter.
Whisk together in a bowl: 1 cup flour, ½ tsp salt, ½ tsp baking powder, 2 tbsp sugar.
Cut in 6 tbsp cold unsalted butter. Add 3 tbsp cold water, make dough, roll into an 11-inch circle, and refrigerate for 30 minutes.
Combine in another bowl: 3 cups rhubarb, 2 tbsp brown sugar, lemon zest, ½ tsp vanilla sugar.
Sprinkle ⅓ cup white sugar over the chilled crust.
Arrange rhubarb mixture over the crust and dot with 1 tbsp butter.
Roll the remaining ng douand gh, and cut it into 1-inch strips. Arrange strips in a lattice pattern on top. Brush with beaten egg and sprinkle with 1 tbsp granulated sugar.
Bake for 45 minutes until golden and bubbling. Allow to cool before serving.