Preheat the oven to 450° Fahrenheit (230° Celsius).
Place butter in a glass 8-inch by 8-inch baking dish. Microwave in 30 second intervals until butter has melted.
In a small mixing bowl, whisk together the dry ingredients: flour, baking powder, sugar, and salt.
Add starter and the milk. Stir until just moistened.
Pour batter on top of the melted butter in the baking dish. Use a spoon or spatula to smooth the batter. Cut the dough into 9 pieces with a knife or bench scraper.
Bake on the second lowest rack in the oven for 15 to 20 minutes until the top has turned golden brown.
Notes
This batter is thick, sticky, and messy. Don’t worry about making these biscuits perfectly smooth when putting them in the baking dish. They’ll expand in the oven.Avoid over mixing, or your biscuits will come out dense rather than fluffy. The less you handle this dough, the better.