Preheat the oven to 410 degrees (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 tops are golden brown, for 12 - 15 minutes. Let scones cool completely.
Omit brushing the tops with milk and sprinkling with sugar.
Whisk together lemon zest and juice, rosemary and milk. Gradually add the icing sugar until the desired consistency is reached.
Drizzle the glaze on the completely cooled scones and serve.
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