Preheat and prep: Heat the oven to 400°F (200°C). Line a baking sheet with parchment paper.

Mix the dry ingredients: In a large bowl, whisk flour, sugar, baking powder, baking soda, and salt until evenly combined.

Cut in the butter: Grate the cold butter on the large holes of a box grater or cut it into small cubes. Work it into the flour with your fingertips or a pastry cutter until you have pea-sized bits. Keep it a little uneven for flakiness.

Stir the wet ingredients: In a separate bowl, whisk the sourdough starter, cream, egg, vanilla, and lemon zest until smooth.

Combine gently: Pour the wet mixture into the dry. Use a spatula to fold just until a shaggy dough forms. If using add-ins, sprinkle them over and fold once or twice to distribute. Do not overmix.

Shape the dough: Turn the dough onto a lightly floured surface. Pat it into a 7–8 inch (18–20 cm) round, about 1 inch (2.5 cm) thick. If the dough is sticky, dust lightly with flour.

Cut the scones: Use a sharp knife or bench scraper to cut the round into 8 wedges. Transfer them to the prepared baking sheet, spacing them out.

Chill briefly: For the best rise and layers, place the tray in the fridge or freezer for 10–15 minutes. This keeps the butter cold.

Brush and sprinkle: Brush the tops with a little cream or milk and sprinkle with coarse sugar if you like.

Bake: Bake for 16–20 minutes, or until the tops are golden and the edges are set. A toothpick should come out clean or with a few crumbs.

Cool and serve: Let the scones cool on the sheet for 5 minutes, then move to a rack. Serve warm with butter, jam, or honey.