Heat oven to 400°F and move rack to lower middle position. Toss sliced rhubarb with 1 Tablespoon of sugar. Spread evenly over a baking sheet lined with a silicone baking mat or parchment paper and roast for 15-20 minutes, stirring halfway through. Set aside to cool. (Keep oven on to cook the scones)
Grate the frozen butter using the large holes of a box grater. Transfer butter to a plate and place in the freezer to keep cold.
Whisk together all purpose flour, whole wheat flour, sugar, baking powder, salt, and baking soda in a large bowl.
Whisk together yogurt, egg and vanilla in a medium bowl.
Stir grated butter into flour mixture until well incorporated. Stir in the yogurt mixture with a spoon or spatula. When most of the flour has been incorporated, transfer to a clean countertop and gently knead until no floury bits are left. Form into a 6x4" rectangle and place in the freezer on a plate for 10 minutes, until slightly chilled.
Place the dough on a lightly floured countertop, and lightly dust the top with flour. Roll into a 12" square. Spread the roasted rhubarb and white chocolate evenly over the dough. Using a dough scraper to help remove the dough from the counter, roll the dough into a log (as if you were making cinnamon buns). Press the log into a 4x12" rectangle and cut into 8 triangles (cut in half, then cut each half in half to form squares, then cut each square in half on the diagonal to form triangles).
Place the triangles on a baking sheet lined with a silicone baking mat or parchment paper, and sprinkle lightly with sugar (use about 1 teaspoon total). Bake for 15-17 minutes, until golden brown on top.