In a big bowl mix together the flour, salt, sugar, baking powder and baking soda.
Add in the cold butter and mix it with your fingers or using a fork until a sand-like dough forms. This has to be done fast so that the butter keeps cold (this step can also be done using a food processor).
In a small bowl mix the egg, sour cream and buttermilk together. Take two tablespoons of this mixture and set aside to brush later the scones.
Add this mixture to the sand-like mixture and mix with a rubber spatula until is all well incorporated. It is vital to just stir and not knead to get better results.
Place the dough on a floured working surface and roll it out with a pin until it is about 3/4 inch thick.
Cut out circular pieces of about 2 inches diameter and place them on a baking sheet previously greased and floured.
Brush the scones with the egg mixture you had set aside before and then bake at 375°C for 15 min. or until they´re golden brown.