Ingredients
1 pound rhubarb, trimmed and cut into 1/2-inch pieces (about 3 cups)
6 ounces fresh raspberries
1/3 cup plus 2 tablespoons granulated sugar
1 teaspoon pure vanilla extract
2 cups unbleached all-purpose flour, plus more for dusting
1 teaspoon baking powder
1/2 teaspoon baking soda
1 teaspoon kosher salt
1 stick cold unsalted butter, cut into 1/2-inch pieces
3/4 cup buttermilk, plus more for brushing
Coarse sanding sugar
Preparation
Preheat oven to 350 degrees. In a bowl, toss rhubarb and berries with 1/3 cup granulated sugar and vanilla. Spread mixture in a single layer on a parchment-lined baking sheet; bake, stirring twice, until juices have thickened but have not burned, 45 to 50 minutes. Immediately transfer to a bowl; let cool completely. (You should have 1 cup. Rhubarb mixture can be refrigerated in an airtight container up to 5 days.)
Increase oven temperature to 400 degrees. Whisk together flour, baking powder, baking soda, remaining 2 tablespoons granulated sugar, and salt. Toss butter in flour mixture to evenly coat, then press between your fingers until mixture has the texture of coarse meal, with a few pea-size pieces remaining. Add buttermilk; stir in with a fork until dough begins to form but mixture is still crumbly. Turn out onto a lightly floured surface; knead just to combine (do not overwork). With a floured rolling pin, roll out to an 8-by-10-inch rectangle, 1/2 inch thick.
Spread rhubarb mixture over dough to edges, then fold tightly into thirds like a letter, sealed-side up. Using a large spatula, transfer to a parchment-lined baking sheet. Brush top with buttermilk; sprinkle generously with sanding sugar. Bake until puffed and golden brown, about 40 minutes. Transfer to a wire rack using parchment; let cool about 30 minutes before serving.
