Pour the melted butter into an 8-inch baking dish, or into a 9-inch or 10-inch cast iron skillet. Place the Bisquick in a large mixing bowl. Add the sour cream. Using a pastry cutter, fork, or two knives, cut the sour cream into the baking mix until little pea-size clumps form.
Pour in the soda and stir just until combined and there are no pockets of dry flour.
Turn the dough onto a well-floured work surface. Flour your hands and fold the dough over onto itself 6-8 times. Pat the dough into a rectangle that’s about 1-inch thick. This is a very soft dough, and can be quite sticky, so keep the counter well-floured, adding plenty of flour to your hands and biscuit cutter as well. Use a floured round biscuit cutter to punch out the biscuits (do not twist the cutter). Arrange the biscuits in the pan with the melted butter. Re-roll the scraps and cut out additional biscuits.
Cover the biscuits with plastic wrap and chill in the freezer for about 15 minutes. Meanwhile, preheat the oven to 425°F. Bake the biscuits until golden brown on top, about 20-25 minutes. Brush the warm biscuits with 1-2 more tablespoons of melted butter.