Rub the frozen stick of butter against the largest grating surface on a box grater or cheese grater, catching the butter in a bowl or plate. Place the butter back in the freezer while you measure the rest of the ingredients.
In a large bowl, whisk together the flour, cheddar cheese, green onions, baking powder, onion powder, baking soda, and salt until combined. Then use a fork to mix in the grated butter until coated.
Make a well in the center of the dry ingredients and add the sour cream. Use the fork to work the sour cream into the dough until it feels hydrated and starts to clump together.
Dump the dough out onto a lightly floured surface and press it together into one large rectangle. Press it down so that it is long enough to fold it in thirds.
Fold the sides of the dough into the center in thirds like a letter. Then press it back down into a rectangle and fold it in thirds one more time. If the dough is sticking to your hands or the surface, dust it with a little bit more flour.
Shape the folded dough into a 9x5 inch rectangle. Dip a sharp knife or bench scraper in flour and cut straight down into the dough lengthwise to cut it in half. Then cut four biscuits from each half, for a total of 8 squares. Dip the knife in flour between cuts to keep it from sticking.
Place the cut biscuits on a lined baking tray. Chill the tray in the freezer while the oven preheats to 450 F/240 C.
Brush the tops of the biscuits with melted butter. Then bake for 14-16 minutes, or until the tops and bottom are golden brown.
Let cool briefly on a wire cooling rack, then serve warm!