1. Prepare Your Dry Ingredients: In a large mixing bowl, whisk together the all-purpose flour, 1/4 cup granulated sugar, baking powder, and salt. Make sure these are thoroughly combined to ensure even leavening.

2. Cut in the Cold Butter: Add the very cold, cubed butter to the flour mixture. Using a pastry blender, two knives, or your fingertips, cut the butter into the flour until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining. This is crucial for flaky scones.

3. Combine Wet Ingredients: In a separate small bowl, gently whisk together the cold sourdough starter, 1/2 cup heavy cream, and vanilla extract.

4. Form the Dough: Pour the wet ingredients into the dry ingredients. Mix with a fork or your hands until just combined. Be careful not to overmix; the dough will be shaggy. Gently fold in the blueberries and lemon zest until evenly distributed.

5. Shape the Scones: Turn the dough out onto a lightly floured surface. Gently pat the dough into an 8-inch round, about 1-inch thick. Avoid overworking the dough. Using a sharp knife or a bench scraper, cut the round into 8 wedges.

6. Prepare for Baking: Carefully transfer the scone wedges to a baking sheet lined with parchment paper, spacing them about 1 inch apart. Brush the tops of the scones with the remaining tablespoon of heavy cream and sprinkle with the remaining tablespoon of granulated sugar.

7. Bake to Golden Perfection: Bake in a preheated oven at 400 F (200 C) for 18-20 minutes, or until the tops are golden brown and a wooden skewer inserted into the center comes out clean.

8. Prepare the Glaze and Serve: While the scones cool slightly on a wire rack, whisk together the powdered sugar and lemon juice in a small bowl until smooth. Drizzle the glaze over the warm scones and let it set for a few minutes before serving.