Preheat your oven to 450 degrees. If you have the convection oven option, use that.
To a large mixing bowl add the flour, baking powder, baking soda, and salt. Next, add the butter (or butter and shortening) to the flour mixture and cut it into the dry ingredients with either a fork or pastry cutter. You want the butter and shortening to be in small pieces and well incorporated throughout the flour mixture. This might take a few minutes to do well.
Once the mixture resembles fine crumbs, toss in the shredded cheddar cheese and the diced jalapenos (if using), set aside.
In a measuring cup or small bowl combine the buttermilk and sourdough starter. Stir until combined. Add the wet ingredients to the bowl with the dry ingredients.
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”. This happens quickly.
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, about 1 inch thick. You could 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.
TIP: Don’t twist the biscuit cutter when you are cutting the biscuits or you won’t get a nice clean edge that will puff up nicely.
Take the remaining dough and knead it slightly together again and cut out more biscuits until the dough is all gone.
Place on an ungreased baking sheet, roughly 2 inches apart.
NOTE: At this point you can cover your biscuits with plastic wrap and let them ferment in the fridge overnight or for 10-12 hours. Then just continue with the recipe as written below. They may need a couple of extra minutes to bake.
Bake biscuits in a preheated oven for 10-12 minutes or until the bottoms are a light golden brown.