Preheat your cast-iron skillet in the oven at 450°F (232°C).
In a large bowl, whisk together the flour, baking powder, baking soda, salt, and sugar until combined.
Add the softened butter and shortening, mixing until the mixture resembles coarse crumbs.
Pour in the buttermilk and gently stir until just combined; a few lumps are fine.
Remove the hot skillet and grease it lightly with butter. Drop spoonfuls of the dough into the skillet, leaving space between them.
In a small bowl, mix the sugar and cinnamon, then sprinkle over the biscuit tops.
Bake for 15-18 minutes, until golden brown and a toothpick comes out clean.
Brush the tops with melted butter after baking for added flavor.