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1
Place rack in the center of oven and preheat oven to 375u00b0F. Line a large baking sheeting with parchment paper and set aside. Dice butter into small pieces and place in the freezer while you prepare the other ingredients.
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2
In a large bowl or the bowl of a standing mixer fitted with the paddle attachment, stir together the flour, sugar, baking powder, cinnamon, baking soda, and salt. Cut the butter in with a fork or pastry blender or place the mixer on low speed. Continue working in the butter until some pieces are the size of oat flakes and some are the size of your thumbnail. The butter pieces may look large, but this is okay. By hand with a spatula, gently fold in the rhubarb.
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3
Whisk together buttermilk and egg yolk. Stir into the dry mixture until a soft, moist dough forms. The dough should be somewhat sticky and not at all dry. If it seems too dry, sprinkle in 1-2 additional tablespoons of buttermilk.
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4
Transfer the dough to a lightly floured work surface, then pat it into a 7-inch disk. Cut dough into 8 wedges. Place the wedges on the prepared baking sheet. For crispier scones, gently separate the wedges. For softer, higher-rising scones, leave the wedges in a circle.
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5
Beat the egg white in a small bowl, then brush over the tops of the scones. Sprinkle generously with sparkling sugar, then bake for 18 to 25 minutes, rotating the pan 180 degrees halfway through. Begin checking at the 18-minute mark (the farther the scones are apart, the more quickly they will bake.) Remove the scones from the oven when they are light golden brown. Cool on a wire rack and enjoy warm or at room temperature.
