Preheat the oven to 220 degrees 425 degrees F.
Sift the flour, bicarbonate of soda (baking soda), cream of tartar, and salt into a large bowl. Using your fingertips, rub in the butter until the mixture resembles breadcrumbs. Add the sugar and mix well.
Set aside about a third of the beaten egg, and combine the rest with the buttermilk, then add the buttermilk mixture to the flour mixture, and mix briefly to combine into a moist dough. Place on a lightly floured work surface and knead ever so slightly to bring together, then press or roll out to a thickness of 3/4-inch.
Using the cutter, cut out approximately 12 scones and place on a floury baking tray. Add about a teaspoon or so of the buttermilk to the remainder of the beaten egg to make an egg wash.
Brush the scones with the egg wash, and dip the tops in sugar if you wish, and bake in the oven for 10 to 12 minutes or until golden brown on top. Eat as soon as possible!
Cinnamon: Add 2 teaspoons of ground cinnamon to the flour.
Orange: Add the finely grated zest of 1 orange to the flour.
Chocolate chip: Add 3 1/2ounces of dark or milk chocolate chips (or chopped chocolate) to the dry ingredients before you add the liquid.
Sultana or raisin: Add 3 1/2-ounces of sultanas or raisins to the dry ingredients before you add the liquid. I also sometimes add 3 1/2-ounces dried cranberries or dried blueberries instead of the sultanas.