2 pounds rhubarb, trimmed, sliced 1″ thick
1 cup sugar
1/2 cup red wine
1 vanilla bean, split lengthwise
Biscuits and assembly:
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
For roasted rhubarb:
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.
For biscuits and assembly:
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.