Preheat oven to 400F (205C).

Combine the sugar, flour, baking powder and salt in a large bowl.

Cut in the butter using a fork until pea-sized bits form.

Combine sour cream and 1 cup of milk in a small bowl, and add to the flour mixture.

Zest the orange, and add the zest to the mixture.  Squeeze 1 tablepoon of fresh juice from the orange and add that to mixture.

Stir until combined, but do not overmix.  The dough formed should be a bit lumpy.

Using floured hands, form the dough into balls about 2 or 3 inches in diameter.  Place each dough ball onto a baking sheet lined with parchment paper or greased with cooking spray.  Lightly flatten each dough ball.

Make an egg wash by whisking together the egg and 1 tablepoon of milk in a small bowl.  Brush the top of each scone with the egg wash.

Bake for 10 to 15 minutes in the preheated oven or until the tops are golden brown.

Remove from the oven and cool in a wire rack before serving.