Preheat your oven to 400°F and line a baking sheet with parchment paper.: The warm, faint scent of the oven as it reaches 400°F signals you are ready to bake, and a lined pan prevents sticking, aiding in even browning. A properly preheated oven delivers immediate spring to the dough, creating a tender interior and crisp exterior. One common mistake is skipping preheating because it shortens bake time and yields uneven rise, so allow the oven to fully reach temperature before baking.
In a large bowl, whisk the flour, sugar, baking powder, xanthan gum (if using), and salt.: As you whisk the dry mixture, you should notice a uniform pale color and a slightly sweet aroma from the sugar. This even distribution of dry ingredients ensures consistent lift and flavor in every scone. A misstep here is failing to thoroughly mix, which can create pockets of baking powder or salt and lead to uneven texture or bursts of flavor.
Grate the butter using a cheese grater and add it in. Use your hands to blend the butter in to the flour mixture until it looks like coarse sand. A pastry cutter works, too.: The grated butter will feel cool and granular against your palms, and the mixture should resemble coarse sand with visible small flakes of butter . Those flecks are crucial because they melt during baking and create pockets of steam, which yield flakiness. Avoid overworking the dough here, since too much warmth from your hands can melt the butter and reduce the flakiness.
In a separate medium bowl, whisk the egg, milk, lemon juice, lemon zest, and vanilla.: Whisking these wet ingredients should produce a smooth, slightly frothy mixture with a bright citrus perfume from the zest. This step hydrates and flavors the dough uniformly, so the acidic lemon juice and aromatics get evenly distributed. A common error is using warm milk or a warm egg, which can soften the butter prematurely and affect texture.
Add the wet ingredients into the dry ingredients and mix. Stir in the blueberries and stir until well mixed.: As the wet meets the dry, the dough will come together, feeling sticky but cohesive. When you fold in the blueberries , do so gently to avoid crushing them, otherwise the dough can become purple and soggy in spots. The goal is an even distribution with visible berries. Overmixing at this stage can activate binders too much, producing a dense result, so mix until combined.
Pour the dough onto the pan and shape into a circle about 8 inches wide in diameter (it will be sticky). Use a sharp knife or bench scraper to cut the dough into 8 portions.: The dough will feel tacky but should hold shape when formed into an 8 inch circle. Press lightly to even thickness, then score into 8 wedges. The visual cue is a smooth, even disk with visible berry pockets. A frequent slip is trying to roll the dough like pastry, which compresses the layers and reduces flakiness, so handle minimally.
Brush the tops with cream and sprinkle with the cane sugar.: Brushing with cream gives a glossy sheen and promotes a golden color as the sugar caramelizes slightly. The cane sugar will cling and create a light crunch after baking. If you brush too vigorously you can flatten the scored edges, so apply a light, even coat to preserve shape.
Refrigerate for at least 15 minutes.: Chilling solidifies the butter again, which helps the scones stay flaky during baking. You will feel the dough firm up when touched, and it is easier to handle. Skipping this rest can lead to spreading and a denser interior because the butter will melt too soon in the oven.
Once chilled, bake until the edges are deep golden brown and the tops are lightly browned, about 30 to 40 minutes, depending on your oven.: During baking you will notice an enticing aroma as the top browns and the blueberries soften. The edges turning deep golden is the key visual cue that the scones are done, and a skewer inserted near the center should come out mostly dry. A common mistake is relying solely on a short timer rather than visual cues, which can result in underbaked interiors or overly hard exteriors.
Remove from oven and let cool completely in the pan.: Cooling on the pan allows the crumb to set and prevents the glaze from sliding off when applied. The scones will lose some of their oven heat and the interior will finish cooking gently. Cutting into them too soon often yields a gummy center, so patience produces the best texture.
Once cooled, whisk together the glaze ingredients and drizzle over the scones.: The glaze should be smooth and glossy, with a bright lemon fragrance, and it will set to a silky finish when drizzled. The contrast of a sweet glaze on a tender, slightly crisp scone is a finishing touch. Lumping the powdered sugar or adding too much liquid makes a runny glaze, so whisk to the right consistency before drizzling.