Prepare the lemon sugar. In a large mixing bowl, add the granulated sugar and lemon zest. Using your fingers, rub them together until the mixture is very fragrant and resembles damp sand. Set the zested lemon aside to use for the glaze.
Add the dry ingredients. Add the flour, baking powder, and salt to the bowl along with the lemon zest and sugar. Stir to combine.
Add the butter. Using a box grater, grate the cold (or frozen) butter into the bowl. Gently incorporate it into the flour mixture using a rubber spatula. The butter should remain visible and coated in flour.
Add the blueberries. Gently fold in the blueberries until they are evenly distributed and coated with flour. Set the bowl aside.
Whisk together the wet ingredients. Add the egg, sourdough discard, and vanilla extract to a separate medium bowl. Whisk well until smooth and fully combined.
Combine the wet and dry ingredients. Add the sourdough mixture to the flour mixture. Using a rubber spatula, gently fold the two together until the dough just starts to come together. It will still look a bit dry.
Add the heavy cream. Pour in the heavy cream and gently fold it in until the dough comes together into a shaggy mass. Avoid overmixing.
Chill the dough. Transfer the dough to a piece of plastic wrap, shape it into a disc, and wrap tightly. Refrigerate (cold ferment) for at least 30 minutes, or up to 24 hours**. We like to do this overnight for about 8-12 hours; it is both convenient and improves flavor.
After chilling, prepare to bake. Preheat the oven to 375ºF (190ºC) and line a baking sheet with parchment paper.
Unwrap the chilled dough and place it on a piece of parchment paper. If the dough feels too wet, lightly sprinkle a small amount of flour over and under the dough and gently rub it in.
Shape the scones. Using a bench scraper (or sharp knife), cut the dough into 8 equal wedges.
Transfer the scones to the prepared baking sheet, spacing them apart so they can bake evenly.
Bake until golden. Bake in the preheated oven for 22-25 minutes, or until the tops are lightly golden, the scones are set, and they reach a minimum internal temperature of 200ºF (93ºC).
Prepare the glaze. While the scones bake, whisk together the lemon juice and powdered sugar until smooth. Cover and set aside.
Cool briefly before adding the glaze. Allow the scones to cool for 10-15 minutes. Then drizzle the scones with the prepared lemon glaze (if using). Serve warm or at room temperature for the best results.