A sweet, summery strawberry butter or fruity jam accompany these subtly tart, rhubarb-buckwheat scones beautifully. A perfect way to break bread Sunday morning (or any day of the week).
Preheat oven to 425 degrees. Toss rhubarb with 1/4 cup granulated sugar; let stand, stirring occasionally, 20 minutes. Meanwhile, whisk together cream and egg. In a large bowl, whisk together both flours, remaining 1/4 cup granulated sugar, baking powder, and salt.
Cut butter into flour mixture until it resembles coarse meal with a few pea-size pieces remaining. Reserve 1/3 cup rhubarb mixture; stir the rest into flour mixture.With a fork, stir cream mixture into flour mixture just until combined.
Transfer dough to a lightly floured surface. Pat into a 1-inch-thick rectangle, about 6-by-7 1/2 inches. Cut into six 2-by-3 3/4-inch rectangles, then cut rectangles on the diagonal into triangles. Transfer to parchment-lined baking sheets; freeze 10 minutes. Brush tops with cream; top each with a few pieces of reserved rhubarb mixture. Sprinkle generously with sanding sugar and bake, rotating once halfway through, until golden, 20 to 25 minutes. Let cool completely; serve with berry butter. Scones are best eaten the same day, but can be stored in an airtight container at room temperature up to 1 day.