Preheat the oven to 400 degrees Fahrenheit. Then prepare a baking sheet by lining it with parchment paper. Set aside.
Cut the butter into small pieces. Place in the refrigerator or freezer to keep it cold while you prepare all the other ingredients.
In a large bowl mix together the flour, sugar, baking powder, and salt. Set aside.
In a medium sized bowl whisk together the sourdough starter and heavy cream until it is fully combined. Then whisk in the egg, lemon juice and vanilla extract.
Using a pastry cutter, cut the butter into the dry ingredients. The butter should be broken up into pea sized pieces.
Fold in the blueberries and lemon zest until the blueberries are completely mixed and coated in the flour.
Using a wooden spoon or spatula, mix the wet ingredients into the dry until it forms a rough dough.
On a lightly floured surface turn the dough onto the counter. Very gently knead the dough into a rough ball. The dough should be very wet and tacky. Use well floured hands to help handle it. If too much flour is added the scones will turn out dense.
Using either your hands or a floured rolling pin, start making the ball into a 6-7" round disc. Make sure it is as even as possible.
Using a bench scraper, divide the dough into 8 triangles.
Keep using the bench scraper to move the scones to the prepared baking sheet.
Brush the top of the scones with heavy cream.
Optional: If you feel the butter has softened and is now warm, place the baking sheet in the refrigerator for 30 minutes before baking. This will stiffen the butter back up.
Place in the oven and bake for 22-25 minutes or until they are light golden brown.
While the scones are baking prepare the lemon glaze by whisking the powdered sugar and lemon juice until smooth.
Remove scones from the oven and place on a wire cooling rack.
Drizzle the lemon glaze on top and enjoy!