Line rimless baking sheet with parchment paper or silicone mat; or dust lightly with flour. Set aside.
In large bowl, whisk together flour, sugar, baking powder, baking soda and salt. Using pastry blender, cut in butter until mixture looks like coarse crumbs. Pour buttermilk over top and stir with fork to form soft, slightly sticky ragged dough.
Turn dough out onto lightly floured surface. With lightly floured hands, knead gently 10 times to smooth dough, adding a sprinkle more flour to the work surface if needed. Gently pat or roll out into generous ½-inch thick round. Using 2-inch cutter, cut out rounds. Place on prepared baking sheet. Gather up scraps and repat dough; cut out more rounds, pressing any remaining scraps into a final scone (the bakers’s scone).
Brush tops of scones with egg. Bake in centre of 425°F oven until golden, about 15 minutes. Let cool on pan on rack for 5 minutes. Transfer to racks to finish cooling.
Tip: Before cutting out scones, dip cutter into flour, repeating the dipping between cuts so cutter doesn’t stick. Always cut out scones, or any cookie, with as little space as possible between rounds as possible. You can reroll the scraps, but at every rerolling the results are tougher. So get as many rounds as you can out of the first roll.