Preheat the oven to 475 degrees F and spray a 9-inch cake pan with cooking spray.

In a medium bowl, whisk together the self-rising flour, baking soda, salt, and sugar. Add the shortening and cut it into the flour using a pastry blender or your fingers until you have pea sized pieces.

In a small bowl, whisk together the buttermilk and sour cream until smooth. Make a well in the center of the flour mixture and add the buttermilk mixture. Stir to combine – do not overmix – and set aside for a few minutes. 

Sprinkle the all purpose flour onto a shallow bowl. Working one at a time, spoon a scant ½ cup batter of the biscuit dough onto the flour. It will be soft and scoop-able. (A lightly greased measuring cup or portion scoop works well for this). Move the soft dollop of dough around in the flour to coat it on all sides, so that you can handle it without it sticking to your hands, then pick it up and shake to remove any excess flour. Position the floured biscuit dough in the pan, and repeat with the remaining biscuit dough, spacing the floured biscuits evenly in the pan. 

If you position the biscuits along the outside of the pan, you can position the last portion of dough in the center and it won’t matter if that last portion is a bit smaller (or larger) than the rest. You should be able to shape 7 biscuits. 

Brush the melted butter evenly over the tops of the biscuits. 

Bake until the biscuits are lightly golden and puffed, usually 16 to 18 minutes. Remove the biscuits from the oven, set aside to cool in the pan briefly, then invert the pan onto a plate and the biscuits should fall right out. Turn them right side up and break them apart. Serve with butter and jam or with fried chicken.