Preheat the oven to 450 degrees.
Cut the cold butter or cut into small cubes and place in the freezer while you measure the other ingredients.
In a large bowl, mix the flours, salt, and baking powder together.
Cut the butter into the dry ingredients by using the tips of your fingers to smear them together or use a pastry cutter or fork. The mixture should look crumbly.
Add the buttermilk and sourdough culture together. Add to the dry ingredients and mix with a spoon (you might need to use your hands too) until the ingredients just come together. Stop mixing after this so you don't over-develop the gluten. The mixture should be messy and not a smooth ball of dough.
Remove the dough from the bowl and lay on a floured surface. Pat it out with your hands in a circle. Fold the dough onto itself three times to create flaky layers (watch the video easy for instruction).
Flour a biscuit cutter well and cut into biscuits. You can also cut them into squares with a knife so that there isn't any extra dough waste.
Place on a parchment-lined sheet pan and brush the tops with buttermilk or melted butter. Bake for 14-17 minutes or until the biscuits are set and have a bit of color on top. It's better to under bake them and check one for doneness. Biscuits become bricks if over-baked!!