Preheat the oven to 450ยบ F. Lightly coat a cast iron skillet with vegetable oil, using a cloth. Only a very thin layer is needed in a well-seasoned skillet.
In a large bowl, sift together the flour, baking powder, baking soda and salt. Mix until well combined.
Grate the frozen butter with a large hole cheese grater.
Add the grated butter to the dry ingredients and combine well.
Add the buttermilk to the dry ingredients and stir with a rubber spatula until a dough begins to form.
Remove the dough to a lightly floured surface and knead lightly 4-5 times, until the dough comes together, and all crumbs are gathered.
Roll out the dough until it's about 1/2 to 1/4 inch thick. Do not overwork.
Using a biscuit cutter or the metal rim of a mason jar, cut out biscuit rounds. Re-form the dough gently and cut out more biscuits until you have 12-14 biscuits.
Place biscuit rounds into the skillet and place in the oven.
Bake for 14-16 minutes, or until golden brown.
Brush the tops of the biscuits with melted butter, remove from the pan and repeat the baking process with the remaining rounds.
Serve warm with more butter and jam, or however you please.
