Preheat oven to 400℉ and line a baking sheet with parchment paper.
In a large mixing bowl, sift together the flour, sugar, baking powder, and salt. Place the bowl in the freezer for 10 minutes. Keeping the ingredients cold helps create flaky, buttery scones.
Use a pastry cutter, food processor, or cheese grater to cut the cold butter into the dry ingredients. Continue until the mixture resembles coarse sand. If you’re using a food processor, pulse it for 1-2 seconds at a time. If the butter starts warming up, pop the mixture back into the fridge for 10 minutes.
Add the heavy cream, sourdough starter, egg, lemon zest, lemon juice, frozen blueberries (do not thaw), and vanilla to the bowl. Stir with a fork until just combined.
Turn the dough out onto the parchment-lined baking sheet and shape it into a disc about 9 inches in diameter. It doesn’t need to be perfect, just keep the thickness even. To avoid smashing the dough, I use my hands instead of a rolling pin. If the dough feels too dry and isn’t holding together, add more cream ½ tablespoon at a time. If it’s too wet, sprinkle in a little more flour one tablespoon at a time.
Use a sharp knife or bench scraper to cut the dough into wedges, then space them about 2 inches apart on the sheet.
Brush the tops of the scones with a small amount of cream. I like to sprinkle the scones with a dusting of sugar! Bake at 400℉ for 20-25 minutes, depending on your oven. The scones should be lightly golden at the edges, and a toothpick inserted into the center should come out clean. Don’t wait for them to get too brown, or they’ll be dry.
Let the scones cool on a wire rack. While they’re cooling, mix the milk, powdered sugar, and vanilla in a small bowl until smooth. Drizzle the glaze over the cooled scones.