In a large mixing bowl, whisk together the gluten-free flour, sugar, baking powder and salt.
Grate the frozen butter using a box grater. Add the butter to the flour mixture and combine with a pastry cutter, two forks, or your fingers until the mixture resembles pea-sized crumbs.
Stir the mix-ins into the flour mixture until evenly distributed.
In a separate bowl, whisk together the sour cream, egg and *vanilla until well-combined.
Using a rubber spatula, stir the sour cream mixture into the flour mixture until no dry bits of flour remain.
Spread out a large piece of plastic wrap. Using wet hands, transfer the dough to the plastic wrap. Clean and wet your hands again then pat the dough to form a 6-inch round that is 1-inch thick.
Wrap the dough tightly in plastic wrap and refrigerate for 20-30 minutes. Then freeze for 15 minutes (or up to 5 days).
Preheat the oven to 500°F. Line a large baking sheet with parchment paper or a silicone baking mat.
After 15 minutes (or up to 5 days), take the dough out of the freezer. Spray a sharp knife with cooking spray and cut the dough into 6 equal wedges. Arrange the scones on the prepared baking sheet. Sprinkle with coarse sugar, if desired.
Place the scones in the oven, then immediately lower the oven temperature to 425°F. (You want the oven temperature to start hot so the scones turn golden brown as they cook through.) Bake for 12-14 minutes until golden brown.
Remove from the oven. Transfer the scones to a wire rack and let cool for 20 minutes before serving.