Preheat the oven to 220°C. Line a baking tray with parchment paper. If your kitchen is warm, place the cubed butter in the freezer for 5 to 10 minutes before mixing. Cold butter is the main structure here. It melts in the oven and creates steam pockets, which gives the scones their flaky, tender interior. That extra freezer time on a warm day is a genuine, practical safeguard — butter that's begun softening even slightly loses some of its ability to create the distinct steam pockets this whole recipe depends on.

In a large bowl, whisk together the flour, baking powder, baking soda, salt, and sugar. Make sure the baking powder and baking soda are evenly distributed. That thorough whisking ensures the leavening agents are dispersed consistently throughout the flour, rather than clumped in pockets that would cause some scones to rise more unevenly than others.

Add the cold butter cubes to the flour mixture. Rub them in with your fingertips or cut them in with a pastry cutter until the mixture looks coarse and crumbly. Some butter pieces should still be visible, about pea-sized. Do not fully blend the butter into the flour. If the mixture looks like fine sand with no butter pieces left, the scones will be more cakey and less flaky. Keeping those visible, pea-sized fat pieces intact, rather than working them in fully, is exactly what preserves the pockets of steam responsible for a genuinely flaky, layered interior once baked.

Add the grated cheddar and sliced chives. Toss through the flour mixture until evenly distributed. Keep a little extra cheddar aside for topping. Reserving a small portion of cheddar specifically for the top, rather than folding it all into the dough, ensures there's cheese remaining to melt into those classic, visible golden patches on the finished scone's surface.

In a separate bowl, whisk together the egg and cold buttermilk. Pour into the flour mixture and stir gently with a fork or spatula until a rough, shaggy dough forms. Add the extra 15 to 30ml buttermilk only if there are dry patches that will not come together. The dough should look rough, not smooth. Overmixing develops gluten and makes the scones tough instead of tender. Stopping the mixing the moment the dough comes together, rather than continuing for a smoother appearance, is what protects the finished scones from toughening — flour's gluten develops more the longer it's worked, and this rich dough has genuinely little need for that extra structure.

Turn the dough onto a lightly floured surface. Press it together gently with your hands. Divide into 2 equal pieces. Pat each piece into a round about 16 to 18cm wide and 2.5 to 3cm thick. Cut each round into 6 wedges, giving you 12 scones total. Press the dough together; do not knead it like bread. A few cracks and rough edges are correct for craggy, flaky scones. Pressing rather than kneading protects the same visible butter pieces built into the dough during the cutting-in stage — kneading like bread dough would work that butter in further and smooth over the cracks and rough texture this scone's whole craggy character depends on.

Transfer the cut scones to the parchment-lined tray, leaving space between them. Chill for 15 minutes. While they chill, whisk together the egg, milk or buttermilk, and a small pinch of salt for brushing. Chilling firms the butter again and helps the scones rise upward instead of spreading sideways. That firmed-up butter is directly what lets the scones hold their shape through the oven's initial heat — dough with softened butter would spread outward before it ever had the chance to rise properly upward.

Brush the tops of the chilled scones lightly with the egg wash. Scatter the reserved cheddar over the tops. Do not let egg wash run heavily down the sides. Egg wash on the cut sides can seal the layers and reduce rise. Brush the tops only. That restraint protects the dough's own cut edges from being glued shut before baking even starts — sealed edges would trap the layers built during shaping, directly limiting how much the scone can actually rise in the oven.

Bake for 16 to 20 minutes, until the scones are deeply golden, risen, craggy on top, and cooked through in the center. The cheese should be melted into golden patches and the edges should feel set. The tops should be golden-brown, not pale. Underbaked scones taste doughy in the center and will collapse slightly as they cool. That deep golden colour reflects real, substantial baking having taken place all the way through — a paler scone pulled too early would still taste raw at its centre despite looking technically finished on the outside.

Let the scones cool on the tray for 5 minutes, then transfer to a rack. Serve warm or at room temperature. Each serving is 3 scones. That brief cooling period on the tray lets the scones firm up just enough to transfer cleanly to a rack without breaking apart, while still being served warm enough for the cheese pockets to stay soft.