01 - Preheat the oven to 400°F. Line a baking sheet with parchment paper.
02 - In a large bowl, whisk together flour, sugar, baking powder, salt, and lemon zest until well combined.
03 - Add cold butter to the flour mixture. Using a pastry blender or your fingertips, cut the butter into the flour until the mixture resembles coarse crumbs. Some pea-sized pieces of butter should remain.
04 - Gently fold the blueberries into the flour-butter mixture, taking care not to crush them.
05 - In a small bowl, whisk together heavy cream, egg, and vanilla extract until fully blended.
06 - Pour the wet mixture into the dry ingredients. Stir with a spatula or wooden spoon until just combined. The dough will be shaggy. Do not overmix.
07 - Turn the dough onto a lightly floured surface. Gently pat into a 1-inch thick circle. Using a knife or bench scraper, cut the circle into 8 equal wedges. Transfer wedges to the prepared baking sheet, spacing them 2 inches apart.
08 - Brush the tops of the scones with a small amount of heavy cream. Bake for 18 to 20 minutes until golden brown on top. Cool on a wire rack for at least 10 minutes.
09 - While scones bake, set up a double boiler. Fill a saucepan with 1 inch of water and bring to a gentle simmer. In a heatproof bowl that fits over the saucepan without touching the water, whisk together lemon juice, lemon zest, sugar, eggs, and salt until smooth.
10 - Place the bowl over the simmering water. Whisk constantly for 8 to 10 minutes until the curd thickens enough to coat the back of a spoon and reaches 170°F. Remove from heat immediately.
11 - Add the butter pieces to the hot curd, whisking until completely melted and smooth. The curd will continue to thicken as it cools.
12 - Let the lemon curd cool to room temperature, about 30 minutes. Serve scones warm or at room temperature with a generous dollop of lemon curd. Store leftover curd refrigerated in an airtight container for up to 1 week.