Preheat oven to 475 degrees F (245 degrees C). Spray a 10 1/2 inch cast iron skillet with non stick cooking spray.
In a medium bowl, combine flour, baking soda, salt and sugar.
Cut in the shortening and softened butter with a fork until the lumps are pea size. Gently stir in the buttermilk until all of the flour is incorporated; do not over mix. The dough should be very wet. If not, add more buttermilk to achieve a sloppy consistency.
Let stand for 2 to 3 minutes.
Place all-purpose flour in a bowl. Spray an ice cream scoop or large spoon with vegetable spray and scoop out dough. Drop each biscuit one at a time into the flour. Using well-floured hands, pick up each piece and pass from hand to hand lightly to shake off excess flour and shape it into a soft round.
Place the biscuits gently into cast iron skillet. They will be a tight fit. Do not mash the biscuits down!
Brush tops lightly with melted butter.
Bake at 475 degrees F (245 degrees C) for 16 to 18 minutes. Cool before serving.
