Line a large baking sheet with parchment paper.

In a large bowl, whisk together the flour, sugar, baking powder, and salt.

On the large holes of a box grater, grate the frozen butter. Add to the flour mixture and toss to coat. Use your hands to work the butter into the flour until the mixture resembles a coarse meal. Add desired mix-ins and toss to incorporate.

In a medium bowl, whisk together the buttermilk, egg, and vanilla, if using. Pour over the flour mixture and use a spatula to mix until a shaggy dough forms. Knead with your hands to incorporate any remaining dry flour. The dough should feel soft but not sticky. If it’s sticky, work in a bit more flour. If it’s dry, drizzle in a little more buttermilk. Form the dough into a ball.

Transfer the dough ball to a lightly floured surface and form it into a 7- to 8-inch disk about 1 inch thick. Slice into 8 equal wedges and transfer to the prepared baking sheet, leaving a little space around each scone.

Freeze the scones for 15 minutes. Meanwhile, preheat the oven to 400°F.

Remove the scones from the freezer and brush with buttermilk. Sprinkle with coarse sugar, if using. Bake for 18 to 27 minutes, or until golden brown on top.

Transfer to a wire rack to cool.