In a large bowl, mix together the flour, sugar, baking powder and salt.
Grate the butter into the bowl.
Gently, using a large spoon, combine the grated butter with the flour mix.
Stir in the sourdough starter.
Knead lightly in the bowl, or on a lightly floured work surface. Depending on how thick or thin your sourdough starter is, you may need to add more flour or a splash or two of water.
Shape the dough into loose a ball.
Roll the dough out 1/2" thick. Cut into 3" circles. Re-roll the cut-offs and repeat.
Place the biscuits on a large baking sheet lined with parchment paper. Cover and let rise for 30-40 minutes in a warm place.
Meanwhile, preheat the oven to 475F.
Lightly brush the biscuits with melted butter right before baking.
Once the biscuits have risen, transfer them into the oven, lower the temperature to 425F and bake for 10 minutes.
Rotate the baking pan and bake for additional 8-10 minutes, or until the tops and the bottoms of the biscuits are golden brown.
Remove the biscuits from the oven, transfer to a cooling rack and let cool for 3 minutes before serving. Brush tops with butter again, if desired, it will help keep them soft longer.
Notes
For a whole wheat version, replace one cup of all-purpose flour with one cup of whole wheat flour, or 3/4 cup whole wheat and 1/4 cup rye flour. Increase the amount of sourdough starter to 2 1/2 cups.