Preheat the oven to 450 degrees. If you have a convection oven setting use that to bake the biscuits.
In a large mixing bowl add the flour, baking powder, baking soda, and salt. Next, add the butter and shortening to the flour mixture and cut into the dry ingredients with either a fork or pastry cutter. You want the butter and shortening to be in small pieces.
Once the mixture resembles fine crumbs add the buttermilk and sourdough starter/discard.
Mix until the biscuit dough starts coming away from the large bowl and then transfer to a clean work surface. It will become shaggy dough and still be “crumbly”.
Once it is on the clean work surface take and knead the biscuit dough by hand 10-12 times. Be careful not to overwork the dough.
Next, pat the ball of dough until it is a circle and about 1 inch thick. You could probably use a rolling pin too, but I find my hands work well and help them not get too thin.
Using a biscuit cutter, cut biscuits and place them on an ungreased baking sheet. Make sure they don't touch. You want to make sure to push the biscuit cutter straight down and back up. Don't twist the biscuit cutter when you are cutting the biscuits.
Take the remaining dough and knead it slightly together again and cut out more biscuits until the dough is all gone.
Bake in a preheated 450-degree oven for 10-12 minutes or until the bottoms are a light golden brown.
Remove from the oven and transfer the biscuits to a wire cooling rack. Serve warm and enjoy!