Preheat your oven to 425°F (220°C).
In a large bowl, combine the flour, baking powder, baking soda, sugar, and salt.
Add softened butter to the dry ingredients and mix until crumbly.
Stir in the sourdough starter and milk until a dough forms.
Turn the dough out onto a floured surface and knead gently for about 30 seconds.
Pat the dough into a 1-inch thick rectangle.
Cut out biscuits using a floured cutter or glass.
Place the biscuits on a parchment-lined baking sheet.
Bake in the preheated oven for 15-20 minutes or until golden brown.
Remove from the oven and let cool slightly before serving.