
Preheat the oven to 425°F. Line a baking sheet with parchment paper or a silicone baking mat.
In a large mixing bowl, whisk to combine the flour, baking powder, sugar, salt, and baking soda.
Cut the butter into small cubes. Add it to the flour mixture and toss to combine. Use a pastry cutter or pinch with your fingers to work the butter into the flour mixture until the butter is in pea-sized pieces. Set aside in a cool place. (Refrigerate if you have the space.)
In a medium bowl, whisk to combine the sourdough starter with the buttermilk or milk.
Add the wet mixture to the dry ingredients and gently stir to bring the dough together. Knead 2 to 3 times in the bowl until the dough comes together in one piece. (If excess flour remains at the bottom of the bowl, add more buttermilk or milk, 2 teaspoons at a time, then gently fold the dough together until it creates 1 piece. The dough will still be shaggy.)
Turn the dough out of the bowl onto a lightly floured surface and press out into a ½-inch thick rectangle. Fold the dough in thirds like a letter, then press to flatten to a ½-inch thick rectangle. Repeat this 2 more times. After the last letter fold, press the dough into a 6x4-inch rectangle, about 1 inch thick.
Cut the dough into 6 equally sized biscuits, 2x2-inches each. (Alternatively, you can use a 2-inch round biscuit cutter to cut rounds, then gently press the excess dough together and re-cut.) If you like, brush the tops of the biscuits with more buttermilk or milk for a golden, glossy finish.
Bake for 16 to 18 minutes, until golden brown. Brush with more melted butter while hot. Serve warm with more butter and/or jam.