Set aside 2 Tablespoons of butter milk 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 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 .
Cool down and enjoy.***