Preheat oven to 425 degrees.
In a large bowl, combine flour, baking power, baking soda, and salt.
Using your fingertips, rub the butter and shortening into the dry
ingredients until coarse crumbs are formed. Make a well in the
center and pour in chilled buttermilk. Mix until mostly incorporated
then add the egg. Continue to mix until the dough just
comes together. It will be sticky.
The dry ingredients, butter and shortening can be mixed with a food
processor. Use the metal blade and mix the dry ingredients.
Add the butter and shortening, then pulse the mix until it forms coarse
crumbs. Pour into a bowl then add the buttermilk and egg as noted
above.
Turn onto floured surface. Dust the top with flour and gently fold
upon itself 5 to 6 times. Press dough into 1/2 - 1 inch thick round.
Cut with 2 inch cutters making sure to press straight down through the
dough. Place biscuits on the baking sheet so that they just touch.
Use scrap dough for more biscuits (these will not be as light).
Bake until tall and light brown on top, 15 to 20 minutes.
Yield: about 12 biscuits (depending on thickness)