Preheat oven to 425°F. Line a baking pan with parchment paper. Melt salted butter in a small bowl and set aside.
Set a box grater on top of a sheet of parchment paper. Use the box grater to quickly grate both sticks of unsalted butter into small strands. Transfer grated to the freezer for 15 minutes.
Meanwhile, combine flour, baking powder, sugar, salt, and baking soda in a large bowl, whisking to combine.
Gently stir butter into flour mixture using only a fork to toss. Slowly drizzle buttermilk into flour mixture, tossing with a fork while you work.
Knead mixture a few times in the bowl before transferring to a floured surface. The mixture will be dry, do not over mix. Pat dough into a 1-inch thick square.
Using a knife or bench scraper, cut the dough into 4 quadrants. Stack the 4 pieces on top of each other, and pat dough back down into a 1-inch thick square.
Cut dough into desired amount of square biscuits (I usually make 9). Transfer biscuits to prepared baking sheet. Brush with melted salted butter, and sprinkle with extra salt (if desired).
Place biscuits in oven, reduce temperature to 400°F, and bake 20 to 25 minutes or until biscuits are a deep golden brown color on the tops and edges. Cool slightly and serve warm.