Prepare the crust: In a food processor, pulse the flour and salt. Add the cold, cubed butter and pulse until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.

Gradually add the ice water, one tablespoon at a time, pulsing until the dough just comes together. Do not overmix. Form the dough into a disc, wrap in plastic, and chill in the refrigerator for at least 30 minutes.

Prepare the filling: In a medium bowl, gently toss the chopped rhubarb with the granulated sugar, cornstarch, and vanilla extract until the rhubarb is evenly coated. Set aside while rolling the dough.

Preheat your oven to 400°F (200°C). Lightly flour a surface and roll out the chilled dough to about 1/8-inch thickness. Use a 5-inch round cutter (or a bowl/plate as a guide) to cut out 6 circles.

Place the dough circles onto a parchment-lined baking sheet. Spoon an equal amount of the rhubarb filling into the center of each circle, leaving a 1-inch border bare. Fold the edges of the dough up and over the filling to create a rustic, free-form tart shape, leaving much of the filling exposed.

Brush the exposed pastry edges lightly with the beaten egg wash. Bake for 20-25 minutes, or until the crust is golden brown and the rhubarb filling is bubbly and tender.

Let the tarts cool slightly on the baking sheet before transferring them to a wire rack. Serve warm, perhaps with a dollop of whipped cream or vanilla ice cream.