Prepare dry ingredients - Sift flour, baking soda and baking powder into a large mixing bowl, add kosher salt and sugar, whisk to combine.
270 grams all-purpose flour, 12 grams baking powder, 2 grams baking soda, 3 grams kosher salt, 100 grams granulated sugar
Cut in the butter - Add cold cubed butter to the flour mixture, work the butter into the flour with your fingertips or a pastry blender until all the butter pieces are about pea and lima bean size.
113 grams unsalted butter
Prepare wet ingredients - Add cold buttermilk and egg to another mixing bowl, whisk to combine. Reserve 2 tablespoons in a small bowl to use as egg wash later.
120 grams buttermilk, 1 large egg
Incorporate sourdough discard - Add sourdough discard and vanilla to the buttermilk mixture and whisk to combine.
5 grams pure vanilla extract, 120 grams sourdough starter discard
Make the dough - Make a well in the middle of the flour mixture and pour in the liquid mixture. Fold with a spatula or wooden spoon until a shaggy dough forms. Cover and rest in the refrigerator for 10 to 15 minutes to allow the dough to hydrate.
Roll and fold the dough - Scrape the dough out onto a well-floured board or countertop, and roll out into a rectangle about ½” thick. Fold the dough in thirds, like folding a letter. Rotate 90 degrees, roll it out again and fold in thirds, at least one more time. Finally, shape the dough into a rectangle about 1” thick. Cut into 8 portions. Put them on a small baking sheet or large plate that fits your freezer and freeze for about 30 minutes.
Preheat the oven to 450°F.
Bake the scones - Place the frozen scones on a lined baking sheet, brush the top and sides with the reserved egg wash, and sprinkle generously with turbinado sugar. Bake at 450°F for 10 minutes. Then lower the oven temperature to 400°F and bake for another 12 to 13 minutes until golden brown.
Turbinado sugar for topping
Cool - Remove from the oven and allow to cool on the baking sheet 5 - 10 minutes before transferring the scones to a wire rack to cool completely. Best served warm.