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 blueberries 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 30 minutes. (Do not refrigerate longer than 30 minutes.) Then, place in the freezer for 15 minutes while the oven preheats.
Preheat the oven to 500°F. Line a large baking sheet with parchment paper or a silicone baking mat.
After 15 minutes, 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 topping with the glaze.