For scones, in large mixing bowl, stir together flour, brown sugar, baking powder, baking soda and salt. Cut in butter (with pastry blender or your hands) until mixture resembles coarse crumbs. Toss rhubarb and lemon zest with flour mixture.
Whisk together egg yolk and sour cream, make a well in the center of dry ingredients and add sour cream and egg all at once. Stir with a fork until combined into a ragged dough.
Coat hands with flour or cooking spray and gather dough together: knead gently about 10 times in the bowl, until fairly smooth. Transfer to parchment paper-lined baking sheet and pat into an even 9" circle.
For streusel, combine all ingredients. Gently pat streusel into surface of dough and cut circle into 12 wedges. Do not separate the wedges. Bake for 20-25 minutes, until cooked through and topping is lightly browned and crisp.
For glaze, combine all ingredients and drizzle over cooled scones.
Notes
Store scones in airtight container for 2-3 days, at room temperature or in the fridge. For best storage, freeze scones and reheat individually for 30 seconds for a fresh-tasting treat anytime.
Recipe by Hidden Ponies at https://hiddenponies.com/2011/07/lemon-rhubarb-streusel-scones/