Adjust an oven rack to the middle position and preheat the oven to 425°F. Whisk together the buttermilk and sour cream in a small bowl.

In the bowl of a food processor, combine the flour, baking powder, baking soda, and salt and process until blended, for about 2 seconds. Scatter the ice-cold butter evenly over the flour and pulse until the largest butter pieces are about 1/4 inch at their widest. Transfer to a large bowl.

Add the wet mixture to the flour mixture and fold with a rubber spatula until just combined. Transfer the dough to a floured work surface and knead until the dough just comes together, adding extra flour as necessary.

With a rolling pin, roll the dough into a 12-inch square. Using a bench scraper, fold the right third of the dough over the center, then fold the left third over so you end up with a 12-by-4-inch rectangle. Fold the top third down over the center, then fold the bottom third up so the whole thing is reduced to a 4-inch square. Press the square down and roll it out again into a 12-inch square. Now, sprinkle cheese and pepitas on your rolled out rectangle and press the pepitas into the dough so that they don't fall out as soon as you fold. Repeat the folding process once more.

Roll the dough again into a 12-inch square. Cut six 4-inch rounds out of the dough with a floured round cookie or biscuit cutter. Transfer the rounds to a lined baking sheet, spacing them about 1 inch apart. Form the leftover dough scraps into a ball and knead gently 2 or 3 times, until smooth. Roll the dough out until it’s large enough to cut out 2 more 4-inch rounds, and transfer to the baking sheet.

I like to pop the biscuits into the freezer until my oven is preheated as I find that they hold their shape better when the go into the oven ice cold.

Before they go in the oven, brush the tops of the biscuits with the melted butter and bake until golden brown and well risen, about 15 minutes, rotating the pan halfway through. Allow to cool for 5 minutes and serve while still warm.