Preheat the oven to 400 °.
In a large mixing bowl whisk together flour, sugar, baking powder, baking soda and salt.
Add the cold butter to the flour and toss to coat.
Using your thumbs and fingers shingle the butter into the flour as directed in the post.
In a smaller mixing bowl beat egg and then add the sour cream and milk or cream to the egg and whisk to combine.
Make a well in the dry ingredients and add the egg /sour cream mixture.
Mix using a fork until the flour is hydrated and the dough just comes together. It will be shaggy.
Turn the dough onto a piece of parchment paper and form a 6-8 inch circle. Don't overwork the dough or pat it smooth.
Cut the dough round into 8 even wedges and put them on a parchment lined baking sheet.
Chill the wedges for 30 minutes in the refrigerator.
Bake the scones in the center of the oven for 15-20 minutes until the outside is risen and golden and the center is done.
When the scones are done cool them completely on a wire rack.
Make a glaze.
Add the confectioner's sugar, 1 TBS milk or cream and 1/2 tsp vanilla in a medium bowl and stir to combine.
Drizzle on top of the cooled scones.
Sprinkle sanding sugar over the scones liberally.
Serve with butter and jam or lemon curd.