Mix Wet Ingredients. In a medium bowl, whisk together sour cream, buttermilk, egg yolk, and vanilla extract until smooth and well combined. Add blueberries into bowl. Set aside.
Prepare Dry Ingredients and Cut in Butter. In the bowl of a stand mixer, whisk together the both flours, baking soda, baking powder, and salt. Add half of the cubed butter and, using the paddle attachment on low speed, mix until the butter is fully incorporated. Remove the bowl and paddle, add the remaining butter, and use the paddle manually to press the butter into the flour until you have pea-sized pieces.Alternative (No Stand Mixer). In a large bowl, whisk together the dry ingredients. Use a pastry cutter to cut the butter into the flour until you have a mix of small and larger pieces. Combine Wet and Dry Mixtures. Pour the wet mixture into the flour/butter mixture and stir gently with the stand mixer on low or with a rubber spatula until just combined. To avoid crushing the blueberries, switch to using your hands or a rubber hand spatula to finish mixing. The dough will be sticky—use food-safe gloves if desired.
Chill the Dough. Transfer the dough onto a large sheet of plastic wrap. Shape it into a round disc or ball, wrap tightly, and refrigerate for at least 30 minutes. If freezing, skip to the "Freezing Instructions" below.
Preheat Oven and Prep Pan. Line a baking sheet with parchment paper and preheat your oven to 350°F (175°C).
Portion and Shape Scones. Unwrap the chilled dough and portion it into roughly 1/3-cup scoops. Place them on the prepared baking sheet, leaving room for spreading (bake no more than 6 per tray). For a more uniform appearance, gently shape them by hand into rough balls, like rustic biscuits. You should get about 10 scones total.
Bake the Scones. Bake for 25–30 minutes, or until the tops are golden brown and the bottoms are a deep, crisp brown. Slightly overbaking is okay—it ensures a crunchy exterior. Let the scones cool on the pan for 10 minutes. If desired, drizzle with maple glaze while still warm. Transfer to a wire rack or enjoy immediately. Maple Glaze. In a small bowl, mix confectioners’ sugar and maple syrup until you reach a thick but pourable consistency. Add more syrup if too thick, or more sugar if too thin. Drizzle over cooled scones with a spoon and let the glaze set.