Preheat oven to 425 degrees and grease a 9-inch cast iron pan. If you don't have a cast iron pan, use a 9-inch cake pan.
In a large bowl, whisk together flour, baking powder, baking soda, salt, and 1 tablespoon sugar.
Scatter butter and shortening pieces on top and use your fingers to rub the butter and shortening in until it resembles coarse meal.
Stir in buttermilk until combined.
Spray a 1/2 cup measuring cup with cooking spray and scoop and drop 6 heaping mounds of dough into prepared pan, placing first one in the middle and the others in a circle around it.
Combine 1/4 cup sugar and cinnamon in a small bowl. Sprinkle half of sugar mixture on top of biscuits.
Bake until golden brown on top, about 20 to 25 minutes. Brush tops with melted butter and sprinkle remaining sugar mixture on top.