Preheat the oven to 400°F and cover a baking sheet with parchment paper.
Add flour, brown sugar, salt, cinnamon, and baking powder in a mixing bowl. Stir to combine.
Incorporate butter by using a cheese grater (large side) or cutting it into the mixture with a fork until pea-sized pieces form. Stir to evenly distribute butter pieces.
Add milk, sourdough starter (make sure your starter is thick and not runny!), vanilla, and egg. Mix gently using a spoon then use your hands to completely incorporate the rest of the flour.
IMPORTANT: Depending on the hydration and consistency of your sourdough starter, your dough may need a little more milk (2 Tbsp - 1/4 cup). Add this only if the dough is too crumbly and won't come together.
Turn out the dough onto a lightly floured surface and fold the dough in half then in half again or until all excess scone mix is incorporated.
Shape dough into a 2-inch high, 6-inch diameter circle with the palms of your hands.
Slice dough into 8 pieces like a pie, placing them 2 inches apart on a parchment-lined sheet pan. (at this point you can continue the recipe or place the sheet pan in the fridge for up to 24 hours).
Continue the recipe by melting 1/2 Tbsp butter for the topping and brush it on top of the scones.
In a small bowl stir together 2 Tbsp brown sugar and 1/4 tsp of cinnamon. Sprinkle this mixture over the tops of the scones until it's all used up.
Bake for 18-20 minutes then place on a wire cooling rack. Enjoy!