FOR THE RHUBARB: Adjust oven rack to upper-middle position and heat oven to 325 degrees. Toss all ingredients in large bowl until evenly combined. Arrange rhubarb mixture in even layer in 13 by 9-inch baking dish. Cover dish tightly with aluminum foil and bake for 20 minutes. Remove dish from oven and carefully remove foil. Stir rhubarb mixture and redistribute into even layer. Continue to bake, uncovered, until paring knife inserted into rhubarb meets little resistance, 30 to 35 minutes longer. Let rhubarb mixture cool completely in dish, about 1 hour. (Rhubarb can be refrigerated for up to 1 day and used chilled.)

FOR THE SHORTCAKES: While rhubarb cools, increase oven temperature to 425 degrees. Line rimmed baking sheet with parchment paper. Process flour, sugar, baking powder, and salt in food processor until combined, about 5 seconds. Scatter cold butter over top and pulse until mixture resembles coarse crumbs and butter pieces are no larger than peas, about 10 pulses. Add buttermilk and pulse until dough forms clumps and no dry flour remains, about 12 pulses, scraping down sides of bowl as needed.

Turn dough onto lightly floured counter and knead briefly until dough just comes together, about 3 turns. Using your lightly floured hands and bench scraper, shape dough into 8 by 4-inch rectangle, about 1¼ inches thick. Using sharp, floured bench scraper or chef's knife, trim no more than ⅛ inch dough from each side of rectangle to create even shape; discard dough scraps. Cut dough in half crosswise into two 4-inch squares, then cut each square into four 2-inch squares, flouring bench scraper or knife as needed.

Arrange dough squares at least 2 inches apart on prepared sheet. (Dough squares can be refrigerated, covered with plastic wrap, for up to 12 hours.)

Brush tops of dough squares with melted butter and sprinkle evenly with remaining 2 teaspoons sugar (¼ teaspoon each). Bake until tops of shortcakes are light golden brown, 10 to 14 minutes, rotating sheet halfway through baking. Transfer shortcakes to wire rack and let cool completely before filling, about 10 minutes. (Fully cooled shortcakes can be wrapped tightly in plastic wrap and stored at room temperature for up to 1 day.)

FOR THE WHIPPED CREAM: Using stand mixer fitted with whisk attachment, whip cream, buttermilk, sugar, and salt on medium-low speed until foamy, about 1 minute. Increase speed to high and whip until stiff peaks form, 1 to 3 minutes.

Using serrated knife, split shortcakes in half horizontally, then place bottoms on serving plates. Divide rhubarb mixture evenly over bottoms, dollop with whipped cream, and top with shortcake tops. Serve immediately.