Preheat oven to 400º. Line a baking sheet with parchment paper and set aside. in a small freezer-safe mixing bowl, combine milk and lemon juice and scraped vanilla bean (or vanilla bean paste) and whisk until vanilla beans have dispersed. allow to sit for 5 minutes then add egg and whisk to combine. Last, add melted butter to the mixture. Do not mix or stir. Place butter/sour milk mix mixture in the freezer for 10 minutes, or until butter has begun to firm.
Meanwhile, in a mixing bowl sift flour, sugar, baking powder, baking soda, and salt. Set aside. When the butter has begun to firm, remove from the freezer and gently stir to break disperse butter into the milk mixture. Butter should have the size and texture of oat flakes in the milk.
Add milk mixture to the dry ingredients and fold together until a dough forms. Turn out onto a clean surface and gently fold the dough onto itself 3-4 times, rotating the dough as you fold. roll or pat the dough into 1-inch thickness and shape into a rectangle. Using a floured knife or bench scraper, cut the rectangle in half width-wise, so you end up with two long bars of dough. Cut each bar on an angle, alternating sides of each slice to create 6 triangle shapes, or cut into 6 squares. Repeat with the second bar of dough. place each scone onto a parchment-lined tray and brush with egg wash.
Place in the oven and bake until risen, slightly browned, about 10-12 minutes. Prepare vanilla bean icing while scones are baking. Remove baked scones from oven and allow to cool for 1 minute before transferring to a cooling wrack. Allow scones to cool for 5-10 minutes before glazing.