Preheat your oven to 180°C / 350°F (160°C fan-forced). Line a baking sheet with parchment paper.
In a large mixing bowl, whisk together the all-purpose flour, baking powder, and salt. Add the brown sugar and whisk briefly to combine.
Add the cold cubed butter to the flour mixture. Using your fingertips or a pastry cutter, rub the butter into the flour until the mixture resembles coarse breadcrumbs with some pea-sized pieces remaining. Work quickly so the butter stays cold — this creates the flaky layers.
Make a well in the centre. Pour in the cold milk and cream. Stir gently with a fork or spatula until the dough just begins to come together. Do not overmix — some dry patches are fine at this stage.
Turn the dough out onto a lightly floured surface. Gently fold it over itself 3-4 times (do not knead vigorously — this toughens the scones). Pat or roll the dough out to about 2cm (¾ inch) thick.
Scatter the blueberries evenly over one half of the dough. Fold the other half over the top to enclose the berries. Gently pat or roll the dough back out to about 2cm (¾ inch) thick, being careful not to crush the blueberries.
Using a floured 7cm (3-inch) round cutter or the rim of a glass, stamp out rounds by pressing straight down — do not twist, as this seals the edges and prevents the scones from rising evenly. Gather any scraps, gently pat together, and cut again until you have 8 scones. Place them on the prepared baking sheet with about 2cm (1 inch) of space between each one.
Whisk together the egg yolk and 2 tablespoons of milk. Brush the tops of each scone lightly with the egg wash — this gives them a beautiful golden colour.
Bake at 180°C / 350°F for 15-18 minutes, or until the tops are golden brown and the scones sound slightly hollow when tapped on the bottom. Transfer to a wire rack and allow to cool for at least 5 minutes before serving.