- 2 pounds rhubarb, trimmed, sliced 1″ thick
- 1 cup sugar
- 1/2 cup red wine
- 1 vanilla bean, split lengthwise
- 1 cup cake flour
- 4 teaspoons baking powder
- 1 tablespoon sugar
- 1 1/2 teaspoons kosher salt
- 1 cup all-purpose flour plus more for work surface
- 3 cups chilled heavy cream, divided
- 1/4 cup (1/2 stick) unsalted butter, melted
- Preheat oven to 350°F. Combine rhubarb, sugar, and wine in a medium baking dish or ovenproof skillet. Scrape in seeds from vanilla bean; add bean and toss to combine.
- Roast until rhubarb is very tender and juices are syrupy, 30-40 minutes, depending on thickness of stalks. Let cool. Discard vanilla bean.
- Preheat oven to 375°F. Whisk cake flour, baking powder, sugar, salt, and 1 cup all-purpose flour in a medium bowl to combine. Add 1 1/2 cups cream; gently mix just until dough holds together.
- Transfer dough to a lightly floured surface and form into a 9×6″ rectangle about 1″ thick. Cut dough in half lengthwise, then cut crosswise 3 times to form 8 rectangular biscuits.
- Arrange biscuits on a parchment paper- lined baking sheet, spacing 1″ apart. Brush tops and sides of biscuits with butter. Bake until golden brown, 18-20 minutes. Transfer to a wire rack; let cool (biscuits can be served warm or at room temperature).
- Beat remaining 1 1/2 cups cream in a medium bowl until soft peaks form. Split biscuits and brush cut sides with remaining melted butter. Fill biscuits with roasted rhubarb and serve with whipped cream.
- DO AHEAD: Rhubarb filling can be made 5 days ahead. Let cool, then cover and chill. Reheat slightly before serving. Biscuits can be made 1 day ahead. Let cool completely and store airtight at room temperature.