Preheat your oven to 220ºC (430ºF) and line a baking sheet with parchment paper.
Grate the butter with a box grater and set aside in the freezer until you're ready for it. It's really important that the butter is super cold!
Place flour, sugar, baking power, baking soda and salt into a large mixing bowl. Mix with a large spoon until well combined.
Take your butter out of the freezer and place into the bowl with the dry ingredients. Cut the butter through the flour. It's best to do this with a dough scraper or spatula, but you can use your fingers if you prefer (but be fast as you don't want the butter to melt - see notes on this).
Now tip the buttermilk and sourdough starter into the butter and flour mix. Bring together with a rubber spatula until it's a dry dough (I start with 80g of buttermilk and add a touch more if I need it).
Tip the dough onto a lightly floured surface and fold it gently until the dough comes together better. You want to create layers in the dough to give your sourdough biscuits that lovely flaky, buttery texture.
Gently roll the dough out to around an inch thick (2.5 cm) and press out rounds using a sharp biscuit cutter or a glass. Don't twist the dough, just press straight down. My biscuit cutter is 2 inches across and I can make 7 biscuits out of the dough.
Place the biscuits into a small baking dish or cake pan lined with parchment paper. If you like a really golden brown top, lightly egg wash your biscuits before placing them into the oven at 220ºC (430ºF) for around 25 minutes (it will vary slightly depending on your oven).
Notes
SOURDOUGH STARTER - you can use either active sourdough starter or sourdough discard. I prefer to use active starter for these, but it's really up to you. I do recommend chilling your sourdough starter or discard to ensure it doesn't warm the butter up when you add them together. Just putting it in the freezer or 15 minutes before you make the biscuit dough will really help.BUTTER - make sure your butter is COLD! In fact, take it from the fridge, grate it and then pop it in the freezer! The colder it is, the better your sourdough biscuits will be.BUTTERMILK - will give you the best flavor and texture, but if you don't have any buttermilk you can use whole milk with a dash of white vinegar. Let it sit for 10 minutes before adding to your sourdough biscuit dough.CUTTING THE BISCUITS - for the flakiest, tallest biscuits, you can cut the dough into four strips and stack them on top of each other before you roll the dough out. This will give you plenty of layers when they rise. Don't twist the biscuit or cookie cutter. Just push the cutter straight down and then back up. You really want something with a sharp edge, so a glass is better than a mason jar if you don't have a biscuit cutter.