Preheat your oven to 220C (430F) and place the stick of butter into a square baking dish or cake pan. Place the dish into the oven so that the butter melts. While the butter is melting, make your dough.
In a large mixing bowl, add the all purpose flour, sugar, salt, baking powder and baking soda. Mix together until well combined.
Make a well in the centre of the dry flour mix and pour in the buttermilk and the sourdough starter. Use a spatula to gently bring the wet and dry ingredients into a wet dough. It will be quite marshmallowy and kind of like a big bowl of airy sourdough starter. Leave the batter to sit until the butter has melted in your dish.
Once the butter has fully melted, remove the pan from the oven and carefully pour in the biscuit batter. Use a spatula to smooth it out as best you can, but it will naturally level out in the oven anyway. The batter is literally swimming in butter - don't stress, the butter will absorb into the batter as it bakes. Trust the process!
Spray both sides of a dough scraper with oil and divide the dough into 9 even pieces. The batter will kind of fill in the lines, but this is ok.
Place the baking dish of sourdough butter swim biscuits into the oven for around 25 minutes or until they are golden brown and risen.
Remove the golden brown sourdough butter swim biscuits from the oven and allow to sit for 10 minutes so that any remaining butter can be absorbed. Use a knife to cut along the marked lines to form 9 biscuits and enjoy!