Preheat oven to 375 degrees F. Line a baking sheet with parchment.
Whisk together flours, sugar, baking powder and salt in a large bowl. Cut in butter with a pastry blender or rub in with your fingers, until the mixture has the texture of coarse meal.
Whisk together buttermilk, 1 egg, and the vanilla. Drizzle over flour mixture, and stir lightly with a fork until dough comes together but a small amount of flour remains in the bowl.
Turn out dough onto a work surface, and gently knead dough once or twice just to incorporate flour. Pat dough into a 1-inch thick round. Cut into 12 wedges. Transfer to prepared baking sheet. Brush with egg wash, and sprinkle with sanding sugar. Bake until golden brown and cooked through, about 22 minutes. Transfer scones to wire rack to cool.
Cake Flour: You can purchase cake flour in most grocery stores or online; if you can't get it, substitute ¾ cup sifted all-purpose flour + 2 tablespoons cornstarch
Buttermilk: This is the key to the scones' phenomenal texture. If you do not have buttermilk and can't get it, use this substitution: Place 1½ teaspoons lemon juice or white vinegar in a liquid measuring cup. Add enough whole or 2% milk to bring the mixture to ½ cup. Stir it, then let it sit for 5 minutes before using.
Mix-ins: See the post above for mix-in ideas. Use 1 cup of fresh fruit or other mix-ins.
Make-ahead: Make the dough in advance, following all instructions until baking. Freeze the unbaked scone wedges for up to 1 month. Remove from freezer and allow to thaw 10-15 minutes before brushing with egg wash, sprinkling with sanding sugar, and baking at 375°F for roughly 22 minutes.
Store: Store uneaten scones in an air-tight container for 1-2 days. If you think it will be longer than 2 days before you finish your scones I highly recommend freezing them.
Freeze: To freeze leftover scones, store in a freezer-safe container for up to 1 month. Thaw and reheat in a 350°F oven for 10 minutes.