Preheat oven to 400 degrees F.
In a large mixing bowl, whisk together the sour cream, baking soda, egg and vanilla.
In a food processor, mix or pulse the flour, sugar, baking powder, cream of tartar, and salt. Cut or process in the butter until mixture is coarse.
Dump the flour mixture into the bowl with the sour cream/egg mixture and stir until moist. Stir in the chocolate chips (or other add-ins). If dough is too moist, add more flour. If it is too dry, add a little milk or cream.
Turn dough out onto a lightly floured surface, and knead briefly. Roll or pat dough into two 6 inch (approximately) rounds. Cut each into 6 wedges (8 wedges for smaller scones), and place them 2 inches apart on the prepared baking sheet. Brush with topping.
Alternatively, pat dough into an inch thick slab and cut out 2 1/2 inch (or whatever size you want) rounds.
Bake 18 minutes in the preheated oven, until golden brown and cooked through. Check at 18 minutes, but it may take longer.
Let the scones cool for at least 10 minutes before serving. This is important. They taste much better after they've cooled and set a bit -- less doughy.