Preheat your oven to 425°F (220°C). Line a baking sheet with parchment. This is one of those recipes where heat matters, so let the oven really come up to temp while you prep.
In a large bowl, whisk together the flour, sugar, baking powder, and salt. Simple start. No need to overthink it.
Grab the cold butter. If you have a box grater, grate the butter on the large holes directly into the dry ingredients. Otherwise, cut into small cubes. Toss to coat so the butter is evenly distributed. The goal: visible flecks. Little butter pebbles help form those tender layers.
Stir in the lemon zest. In a measuring cup, combine the cold buttermilk and vanilla (if using). Pour most of the buttermilk into the bowl and use a fork to mix until the dough looks shaggy—still floury in spots, but clumping. If it’s too dry, sprinkle in the remaining buttermilk. Stop once it holds together. Messy is fine.
Toss the blueberries with 1 tablespoon flour in a small bowl. This helps keep them from bleeding too much and sinking. If using frozen blueberries, don’t thaw. Work quickly to keep their color where it belongs.
Tip the shaggy dough onto a lightly floured counter. Scatter the floured blueberries over the top. Gently fold the dough over itself a few times to tuck the berries in, like you’re folding a letter. Try not to mash the berries. A little streaking is okay (looks a bit like tie-dye—pretty).
Pat the dough into a rectangle about 3/4-inch thick. Now do a quick lamination: fold the rectangle in thirds (like a brochure), turn it 90 degrees, and pat back out to 3/4 inch. Repeat once more. This creates simple layers without any drama.
Use a sharp biscuit cutter (2 1/2 to 3 inches) to stamp out biscuits. Press straight down and up—no twisting. Twisting seals the edges and can block the rise. Gather scraps, gently press together, and cut again. Or, if you want zero scraps, slice the dough into squares with a knife. Rustic is welcome here.
Arrange biscuits on the prepared sheet, close but not touching (about 1/2 inch apart). Brush tops with a touch of buttermilk or milk and sprinkle with coarse sugar. That tiny crunch on top is lovely.
Bake for 16–18 minutes, until the biscuits are puffed and golden on the edges. The blueberries will be bubbling a bit. Your kitchen will smell like warm butter and summer.
If glazing, whisk powdered sugar with just enough lemon juice or milk to make a spoonable glaze. Drizzle over warm biscuits and let it set for a minute. If not glazing, a pat of butter and a swipe of jam is never a bad idea.
Serve warm. If you’re making them ahead, cool completely, store airtight, and reheat in a 325°F (165°C) oven for 8–10 minutes. They spring back nicely.