In a large bowl, mix the flour and salt. Cut in the butter until the mixture resembles coarse crumbs. Gradually add ice water, 1 tbsp at a time, until the dough comes together. Divide into two balls, flatten into disks, wrap in plastic, and refrigerate for 30 minutes.

In another bowl, combine the rhubarb, sugar, cornstarch, vanilla powder, and lemon juice. Mix well and let sit for 15 minutes to allow the flavors to meld.

Roll out one dough disk to fit a 9-inch pie dish. Transfer the dough to the dish and trim the edges. Pour the rhubarb filling into the crust. Roll out the second dough disk and place it over the filling. Trim and crimp the edges to seal. Cut slits in the top crust to vent.

Preheat the oven to 375°F (190°C). Brush the top crust with the beaten egg. Bake for 45-50 minutes, or until the crust is golden and the filling is bubbly. Let cool before serving.