Preheat the oven to 400°F and line a baking sheet with parchment paper.
Sift flour, baking soda and baking powder into a large mixing bowl, add kosher salt and sugar, whisk to combine.
Add cold cubed butter to the flour mixture, work the butter into the flour with your fingertips or a pastry blender until all the butter pieces are about pea and lima bean size.
Add egg, buttermilk and vanilla extract to a small bowl, whisk to combine. Make a well in the middle of the flour mixture and pour in the liquid mixture, reserving 1 - 2 tablespoon to use as egg wash later. Fold with a spatula or wooden spoon until a shaggy dough forms.
Turn the dough out onto a well-floured board or countertop, and pat the dough out into a rectangle about 1/2” thick. Fold the dough over itself and pat it out again, do this about 3 - 4 times. Then shape the dough into a circle about 1” thick. Cut into 8 portions.
Place the scones on the parchment-lined baking sheet, brush the top and sides with reserved egg wash, and sprinkle generously with turbinado sugar. Bake for 20 minutes until golden brown.
Remove from the oven and allow to cool on the baking sheet 5 - 10 minutes before transferring the scones to the wire rack to cool completely. Best served warm.