For the Cornmeal Pâte Brisée Crust:
2 cups all-purpose flour
½ cup cornmeal
1 teaspoon salt
1 teaspoon granulated sugar
1 cup (8 ounces) unsalted butter, cold, cut into small pieces
¼ to ½ cup ice water
For the Rhubarb Filling:
1 to 1½ pounds rhubarb (about 5 cups ½-inch diced)
½ cup granulated sugar
¼ cup light brown sugar
¼ cup cornstarch
¼ teaspoon salt
½ teaspoon vanilla extract
1 egg, lightly beaten (for the egg wash)
Sanding sugar or granulated sugar, for sprinkling
1. Make the crust. Place the flour, cornmeal, salt and sugar in the bowl of a food processor, and pulse several times to combine. Add the butter, and process until the mixture resembles coarse meal, about 10 seconds. With the machine running, pour the ice water through the feed tube in a slow, steady stream, until the dough just holds together (do not process for more than 30 seconds). Turn the dough out onto a clean work surface. Divide in half, and place each half on a piece of plastic wrap. Flatten each to form a disk. Wrap, and refrigerate at least 1 hour.
2. While the crust chills, make the rhubarb filling. Combine the diced rhubarb, both sugars, the cornstarch and salt in a medium saucepan and place over medium-low heat. Cook, stirring occasionally, until the rhubarb releases its liquid and begins to breakdown, creating a thick, chunky sauce, about 10 minutes. Remove the pan from the heat and stir in the vanilla extract. Set aside to cool to room temperature.
3. One at a time, roll each disk of dough into a 14-inch circle. Using freehand or an upside-down bowl, cut out 5-inch circles and place on a large, parchment-lined baking sheet. Refrigerate for at least 1 hour. Meanwhile, preheat the oven to 375 degrees F.
4. Spoon rhubarb filling into the center of each dough round (about 2 big spoonfuls), leaving about an inch border around the edge. Fold the border over the rhubarb mixture, overlapping where necessary and pressing gently to adhere the folds.
5. Brush the edges of the dough with egg, and sprinkle with sugar. Bake until the crust is golden brown and the filling bubbles a bit, 20 to 25 minutes. Serve warm or at room temperature. Top with whipped cream or vanilla ice cream.
*Note: The dough can be frozen, wrapped well in plastic, for up to three weeks before using.