In a large bowl pour the sugar and — using the rasp — grate the lemon zest on top;
Using your clean hands, work the zest into the sugar;
Add flour, salt and baking powder and whisk everything together;
Dice the butter, add it to the above mix, and work it into the flour mixture with a pastry blender or by pressing the cubes between your fingers until they are about a pea size;
Add the blueberry into the mix and gently mix them in;
In a small bowl whisk the cold cream with a cold egg and using a silicone spatular fold slowly to the dry mix, until the mixture is mostly moistened, without overmixing it (to avoid activating the gluten);
Put the dow onto a counter — sprinkled with flour — and using your floury hands form a ball without kneading it. Then, pat it down into an 8 in disk (20 cm);
Using a floured knife, cut into eight wedges (triangles, scones), place them on a tray with baking paper beneath, and freeze for a minimum of 20 minutes;
Preheat the oven to 200℃
Take the scones out of the freezer, brush them lightly with more cream, and sprinkle some sea salt on top;
Bake the scones until the tops are lightly browned (20-25 minutes);
Let them cool for 10 minutes on a wire rack and serve.
Notes
If the scones are frozen for longer, give them an extra few minutes in the oven;
If the butter is solidly frozen, you could grate it instead of dicing it;
Substitute the milk butter with vegan butter, the egg with chia egg, and the cow cream with coconut cream for an incredible vegan variant;
Use any given berry instead for your scones (even frozen), and use common sense to mix them in the dow.