Preheat oven to 450F

In a large mixing bowl combine: all purpose flour, baking powder, baking soda, kosher salt.

Using the large holes of a cheese grater, carefully grate the frozen stick of butter. Do not allow butter to sit and soften, must stay very cold. Flour butter stick for better grip or use paper towel. When down to a nub, use a knife to cut into small pieces. [see note #2 for food processor way to prep butter]

Using a silicone spatula, immediately stir grated butter into the flour mixture. Slowly begin pouring in buttermilk and stir with spatula until just combined.

Lightly flour a clean and even surface. Dump dough onto surface. Knead just until no loose flour remains (will not be smooth). Flour a rolling pin and roll the dough out into a large square. Using a pastry scraper, cut the dough into 4 large squares, then stack them on top of each other.

Repeat rolling out the dough, cutting into 4 sections, stacking and rolling out a total of 3 times.

Cut the dough into 9 even squares and place in a greased cast iron skillet. Brush the tops of the biscuits with melted butter and place in the oven.

Bake for 17-19 minutes, rotating the skillet halfway through, until the tops are golden brown. Allow the biscuits to cool for 5 minutes before enjoying.