Add butter and shortening to the dry ingredients and work into flour with a pastry cutter or a fork (or even your fingers) until the flour resembles course crumbs.
Mix milk and sour cream.
Make a well in the center of the flour and pour in the milk and sour cream mixture.
Stir very lighly, only until mixed. DO NOT OVER STIR! This will result in tough biscuits. As soon as a dough ball forms, stop stirring. The dough will be very, very sticky. This is good. It means very moist biscuits.
Turn the dough ball onto a very well floured surface, dust the top of the dough ball with flour and gently fold the dough onto itself 5 or 6 times.
Press down or use a rolling pin to make the pile of dough 1" thick. Use a two inch cutter to cut out 12 biscuits. If you reuse the scraps the biscuits from the scraps will not be as tender as the original batch.
Place biscuits on a baking pan (any kind; cookie, cake pan, etc.) so that the biscuits are just touching if you'd like the sides tender. Make sure there is an inch between biscuits if you'd like crispier sides.
Freeze or Bake 9 - 20 minutes (The time depends on the size of your biscuits so keep an eye and note how long it takes for the size and temperature of your oven. If making small biscuits it will take much less time).
You may brush the tops with milk or butter for a golden brown biscuit.
These can be frozen and baked at this stage (takes a bit longer, just keep an eye on them), and they still come out perfect. I quadruple the batch and freeze a bunch for weekday breakfasts.
If you want a warm biscuit just pop them in the microwave or back into the oven and they taste like they are fresh from the oven.