Set aside 2 Tablespoons of buttermilk in a small bowl for brushing.
In a bowl, mix egg, buttermilk and vanilla. Set aside.
In another bowl combine first 6 ingredients then mix well.
Add grated frozen butter to the dry ingredients and mix.
Add the rhubarb and mix well until everything is evenly coated.
Add the buttermilk mixture and mix with a spatula until flour disappears, the dough will be very sticky.
Dust a clean working surface with some flour and dust your hands as well.
Turn the dough into the floured surface and dust each side with about 1 Tablespoon of flour just enough to be able to handle the dough. Do not be tempted to increase flour.
Cut the dough in half and press each half into a 6 inch circle.
Cut each half into 8 wedges.
Place on a baking sheet lined with parchment paper and freeze for 20-30 minutes.
Preheat oven to 400F. Arrange two baking racks one on the upper third and the other on the lower third of the oven.
Take the scones out of the freezer, brush top with buttermilk and sprinkle some coarse sugar, if desired.
Bake 15-20 minutes on the lower third or until the bottom starts to brown then move up for 5-10 more minutes or until top is golden brown.