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Preheat oven to 450°F.
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Combine flours, sugar, salt, baking powder, baking soda, and zest in a medium bowl. Cut in cold butter.
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In a small bowl, combine buttermilk, 1 egg, 1 egg yolk, and vanilla.
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Form a well in the dry ingredients, add wet ingredients. Combine, but do NOT overmix.
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Form into a 6x9" rectangle, cut in half lengthwise and thirds widthwise, cut each square in half diagonally. Place on baking sheet (preferably lined with a silicone mat or parchment).
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Beat remaining egg white with 1 tsp water. Brush egg wash on scones, dust with coarse sugar if desired.
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Bake about 12 minutes or until golden brown.
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Freeze leftovers immediately.