In a large bowl mix together the dry ingredients: flour, salt, sugar, baking soda and baking powder until well combined.
Grate 1 stick of cold butter using a large box grater or cut the butter into very small cubes.
Add half of the butter to the flour mixture. Work the butter into the flour using a pastry cutter or your hands until the mixture resembles coarse crumbs.
Add the milk and sourdough discard to the dry mixture and stir to combine. The dough will be shaggy and sticky.
Turn out the dough on a lightly floured surface and use your hands to press in to a 1 inch thick rectangle.
Take the reserved butter and sprinkle half on the top of the dough rectangle.
Fold the top of the biscuit dough over to cover the butter. Sprinkle on the remaining butter and fold the dough over again to encase the butter.
Use a rolling pin to roll the dough out to 2 inches thick.
Use a biscuit cutter to cut out biscuits. Place the biscuits in the fridge on a baking tray while you preheat the oven to 450 degrees.
When the oven has preheated, remove the biscuits from the fridge and bake at 450 degrees for 14-16 minutes or until golden brown and cooked through.