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, baking soda, salt, and lemon zest until well combined.
03 - Add the cold butter cubes to the flour mixture. Cut in using a pastry blender or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
04 - In a small bowl, whisk together buttermilk, egg, and vanilla extract until blended.
05 - Pour the wet mixture over the flour mixture. Stir gently with a spatula until just combined; do not overmix.
06 - Fold in the fresh blueberries carefully, taking care not to crush them.
07 - Turn the dough onto a lightly floured surface. Gently knead once or twice, then pat into a 1-inch thick circle. Cut into 8 equal wedges.
08 - Transfer wedges to the prepared baking sheet. Brush tops with cream or milk and sprinkle with coarse sugar if using.
09 - Bake for 18-20 minutes until golden brown. Transfer to a wire rack to cool slightly.
10 - In a heatproof bowl, whisk together lemon juice, lemon zest, sugar, and eggs until smooth.
11 - Set the bowl over a saucepan of simmering water to create a double boiler. Whisk continuously for 8-10 minutes until thickened and the mixture coats the back of a spoon.
12 - Remove from heat. Whisk in butter pieces one at a time until smooth and melted. Transfer to a clean jar and let cool completely.
13 - Serve scones warm or at room temperature with a generous dollop of lemon curd on the side.