Line a baking sheet with parchment paper and set aside.
In a small bowl, whisk together the cold sourdough discard, chilled pumpkin purée*, egg, and vanilla extract until smooth. Place the bowl in the refrigerator (we want to keep the ingredients as cold as possible!).
In a large bowl, whisk together the flour, sugar, baking powder, spices, and salt. Set aside.
Grate the frozen butter on the large holes of a box grater. Be careful with your fingers as you're doing this!Add the grated butter to the flour mixture, and incorporate using a pastry cutter, forks, or your hands. Press the butter into the flour until you have a coarse mixture with approximately pea-sized butter pieces.
Pour the cold liquid mixture into the flour mixture and use a large spatula to stir until a rough dough forms. It will seem really dry, but resist adding more liquid at this point. Transfer the dough to a work surface, then gather and press it together, lightly kneading until the dough comes together into a ball and no dry spots remain. The dough will feel slightly sticky, but not to the point that it's sticking to your hands and hard to work with. Gather the dough into a ball. If your dough seems too sticky, add more flour 1 Tablespoon at a time until the dough seems manageable.
Lightly flour your work surface, then flatten the dough into a disk about 8 inches in diameter. Use a sharp knife to cut the dough into 8 roughly equal-size pieces. Transfer to the prepared baking sheet. Place the baking sheet with the dough in the freezer for 20 minutes while you preheat the oven.
Preheat the oven to 400℉ (200℃).
After freezing, brush the scones with egg wash, then sprinkle with turbinado sugar. Bake for 18-22 minutes, or until set and browning at the edges (the internal temperature of the baked scones should be 200℉ / 95℃).
While the scones cool, make the glaze. In a small bowl, whisk together the confectioner's sugar, milk, and pumpkin purée. Drizzle the glaze over the cooled scones and serve.
Notes
1 serving = 1 scone. To make this recipe with active sourdough starter, replace the sourdough discard with active starter (200g). You do not need to change any other ingredients in the recipe.