A rich and flaky scone, bursting with rhubarb. Loosely adapted from The Best Quick Breads, by Beth Hensberger.
Preheat the oven to 425 F. Line a large baking sheet with parchment paper.
Toss the chopped rhubarb with 2 tablespoons granulated sugar, and spread on the prepared baking sheet. Roast the rhubarb for 15-20 minutes, or until the liquid released by the rhubarb is bubbling and just beginning to caramelize.
Transfer the roasted rhubarb to a plate to cool, leaving behind the sticky rhubarb juices. (You can discard this rhubarb syrup or reserve it for another use.)
Once the rhubarb has cooled to around room temperature, begin making the scones: In a large bowl, whisk together the flour, baking powder, baking soda and salt. Add the remaining two tablespoons of granulated sugar and 3 tablespoons of brown sugar, and whisk to combine. Place the bowl in the freezer for five minutes to chill.
Using the large holes on a grater, grate the frozen butter into the flour mixture, tossing the mixture occasionally to distribute the butter. If the butter or mixture starts to warm up, stick it back in the freezer for a couple of minutes, and then continue. After you have grated in all the butter, place the bowl in the freezer.
While the flour-butter mixture chills in the freezer for a few minutes, pour the buttermilk into a liquid measuring cup or other small spouted vessel, and whisk in the vanilla.
Remove the flour-butter mixture from the freezer, make a slight well in the center, and pour in the vanilla-flavored buttermilk. Using a rubber or silicone spatula, toss and fold the mixture to incorporate the liquid. When the buttermilk is mostly incorporated, use your hands to knead the mixture into a rough mass, then turn the dough out onto a lightly floured piece of parchment paper and knead just a few times to bring the mixture together into an evenly moistened dough. Lightly flour the top of the mound, and press or roll the dough into a rectangle of about 9 by 12 inches.
Spread the roasted rhubarb evenly over the dough rectangle. Now, fold the rectangle into thirds, like a business letter, encasing the rhubarb, then gently press or roll the folded dough back into a rectangle about 1 1/4 inch thick.
Transfer the dough rectangle, still on the parchment, to a baking sheet, and place in the freezer for 30-45 minutes.
Remove the semi-frozen dough rectangle from the freezer, and place it, still on the parchment, on the counter or table. Line another large baking sheet with parchment paper.
Use a lightly floured 2 to 2 1/2-inch diameter biscuit cutter to cut 10-12 scones from the dough. (To cut out the scones, press directly down with the biscuit cutter, without twisting.) Place the scones at least an inch or two apart on the freshly lined baking sheet. Once you have cut out the first 6 or 8 scones, you'll need to press together the scraps to cut out the remainder. After pressing the scraps together (and/or re-rolling with the aid of some parchment), you may want to place the dough back in the freezer for a few minutes before cutting out remaining scones.
At this point, the scones can either be topped with turbinado sugar and baked, or frozen for future use.
If you want to bake the scones later, place the baking sheet with the cut-out scones in the freezer for at least 2 hours, until the scones are frozen solid, and then transfer the frozen scones to a freezer bag for storage. When you're ready to bake the scones, remove them from the freezer, place them back on a parchment-lined baking sheet, preheat the oven to 425 F, then proceed as below.
When you're ready to bake the scones: If desired, brush the tops of the unbaked scones with heavy cream. Whether or not you first brush the tops with cream, sprinkle on some turbinado sugar.
Place the scones in the 425 F oven, and reduce the oven temperature to 400 F. Bake until risen and light golden brown, which will probably take about 23-25 minutes if your scones were previously frozen. Remove from the oven, allow to cool on the pan for a couple of minutes, and then eat!