Preheat the oven to 220 °C.
Sift the flour together with the baking powder into a large enough bowl, keeping the sieve up so that as much air as possible gets under the flour. Then add the mustard powder, salt and cayenne pepper.
Mix the ingredients well, then grate the butter into the dry ingredients with your fingertips until you have a crumbly mixture.
Now add the grated cheese together with the chives. Mix the egg with two and a half tablespoons of buttermilk and pour it into the dry ingredients. Mix the ingredients first with a spreading knife and finally with your hands to form a soft dough. If the dough is a little dry, add another half tablespoon or so of buttermilk until you have a soft dough that pulls away from the sides of the baking bowl. It is important not to stir the dough too long, or the scones will be heavy.
Roll out the dough as evenly as possible to two and a half inches thick – it should never be thinner than that, therein lies the secret to well-risen scones. Using the cookie cutter, cut out six scones. You may need to fold the dough sections over from scratch and roll them out again to keep six scones.
Place the cut out scones on the greased cookie sheet and brush the top with milk. Then sprinkle with the grated cheese and a generous pinch of cayenne pepper.
Place the scones on the o