When buttermilk biscuit dough gets the jelly roll treatment, this delicious hybrid is the result. The jammy fruit filling is made in the oven, not on the stovetop.
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.