Heat the oven to 450ºF with a rack in the center. Line a sheet pan with parchment paper.

In a medium bowl, use a fork to stir together the starter discard and the sour cream until combined. Place the refrigerator.

In a large bowl, whisk together the flour, baking powder, sugar, salt, and baking soda.

Toss the butter cubes into the flour mixture to coat them in the powder. Use your fingers to press the butter into thin shards, then continue to work it into the flour mixture until it forms chickpea and lentil-sized pieces.

Drizzle the wet mixture over the sourdough mixture, using a fork to mix together into a shaggy dough. Knead the dough a couple times in the bowl just to bring it together.

Dust a work surface with more flour. Dump out the dough. Gently pat the dough into a 1-inch-thick square.

Use a floured knife or bench scraper to cut the dough square into quarters. Stack the quarters on top of each other, then use your hands or a lightly floured rolling pin to press them down into a 1-inch-thick square. Repeat.

Use your hands or a lightly floured rolling pin to gently press and roll out the dough into a 1 ?½-inch thick rectangle. Use a floured knife or bench scraper to carefully trim the outer edge of the dough, re-flouring after each cut. Cut the rectangle into 6 even pieces, re-flouring after each cut. Gently press together the trimmed pieces to make one more biscuit, which will bake off a little less uniform but no less tasty.

Divide the biscuits evenly on the prepared sheet pan.

Bake the biscuits until puffed and deeply golden brown, 13 to 15 minutes. Remove from the oven and let cool for about 10 minutes, then serve warm.