Preheat the oven to 400 °F (200 °C). Line a couple of baking sheets with parchment paper.
Mix together the flours, baking powder, salt, and sugar in a large bowl with a whisk.
Drop in the butter and work it in with your hands until you get a coarse crumb. Stir in the rhubarb.
Add the cream and vanilla, and mix it in with a fork to form a rough dough. Don't overwork it.
Drop the dough onto a lightly-floured work surface and work it with your hands just enough to gather in the dry bits and incorporate them. You can pat out the dough and fold it over a couple of times to create a little layering.
Pat out the dough to about ¾–1 inch thick and cut with a 2-¼" cookie cutter.
Transfer the scones to the baking sheets. You can press the scraps together gently to make a few extras and use up all the dough.
Brush the scones with a little cream and sprinkle with turbinado.
Bake for about 25 minutes until the bottoms begin to turn nice and golden. Let cool slightlly before serving with a little whipped cream and jam.
Notes
You can also use this recipe to make 6 regular sized scones, by patting the dough into a disk and cutting into wedges. Note the baking time will be more, closer to 25 or even 30 minutes.