Preheat the oven to 410°F.
Line a large cookie sheet with parchment paper.
In a food processor, add the flour, baking powder, sugar and salt. Pulse until combined.
Spread the butter on top of the dry ingredients. Pulse until the mixture resembles fine crumbs.
Add the rosemary and lemon zest and pulse until combined.
Transfer to a large bowl.
Add the buttermilk and egg over the flour mixture and stir, using a rubber spatula, until just combined.
Flour a work surface and transfer the dough on it.
Knead for 1 minute.
Using your hands, pat the dough into a circle and cut into wedges.
Arrange scones in a single layer on the prepared cookie sheet.
Brush tops with milk, then sprinkle with sugar.
Bake until the tops are golden brown, for 12-15 minutes.