Combine the flour, sugar, baking powder, cinnamon, and salt in a large mixing bowl and whisk together.
Add the grated frozen butter, toss in the flour mixture, and then use your hands to break up the butter into pea size pieces.
Make a well in the center of the dry ingredients, and in the well, add the sourdough starter, egg and egg yolk, vanilla, and milk. Mix the wet ingredients together and then stir them in with the dry ingredients until just combined, avoiding overmixing.
Place the scone dough in the refrigerator to chill while you prepare the filling.
To prepare the filling, cream the butter until smooth. Add the brown sugar and cinnamon and mix until well combined.
Remove the scone dough from the refrigerator and roll it out into an 8 inch by 12 inch rectangle. Spread the filling evenly over one half of the dough.
Fold the half without filling over the half with filling to make a smaller rectangle. Then, using a bench scraper or sharp knife, cut your dough into four quadrants. Divide each of these quadrants into half diagonally to create 8 triangular scone-shaped pieces.
Cover the scones (with plastic wrap or something air-tight) and allow to ferment for 8-72 hours.
When ready to bake, preheat the oven to 425 degrees Fahrenheit. Then, remove the baking sheet from the refrigerator, uncover the scones, and bake for 17-20 minutes, or until lightly golden on the tops and edges. Remove and allow to cool to room temperature.
Meanwhile, combine the softened cream cheese and butter and mix with a hand mixer until creamy. Add the powdered sugar, salt, vanilla, and milk and mix until smooth and creamy. Put cream cheese mixture into a piping bag (or a plastic storage bag with the corner clipped) and top the scones. Enjoy!