Place 8 tablespoons butter in the freezer for 15 minutes. Using the large holes of a box grater, grate the frozen butter directly into a bowl and return to the freezer until ready to use. This creates the flakiest texture without overworking the dough.

Preheat oven to 400F. Line a large baking sheet with parchment paper. In a large bowl, whisk together the flour, granulated sugar, baking powder, salt, and 1 teaspoon cinnamon. Add the grated frozen butter and toss with your fingertips until coated in flour - do not overmix.

In a small bowl, whisk together 3/4 cup cold cream, the egg, and 1 teaspoon vanilla. Pour over the flour mixture and fold with a rubber spatula until just combined and no dry streaks remain. The dough will look shaggy and slightly sticky.

Mix the brown sugar, 1 1/2 teaspoons cinnamon, and 1 tablespoon melted butter in a small bowl to form a paste. Turn the dough onto a lightly floured surface and pat into a 10x6 inch rectangle. Crumble the cinnamon mixture evenly over the surface, then fold the dough in half lengthwise and pat back into a 1-inch thick rectangle. The swirl will be streaky and imperfect - that is exactly what you want.

Cut the rectangle into 8 wedges using a sharp knife or bench scraper. Transfer to the prepared baking sheet, spacing 2 inches apart. Brush tops lightly with cream and refrigerate for 10 minutes while the oven finishes heating. This firms the butter for better rise.

Bake for 16-18 minutes until the scones are golden brown on top and firm to the touch. The bottoms should be nicely browned. Cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.

Whisk together the powdered sugar, 2 tablespoons cream, and 1/4 teaspoon vanilla until smooth, adding more cream 1 teaspoon at a time if needed for a thick but pourable consistency. Drizzle over the cooled scones and let set for 10 minutes before serving.