Preheat oven to 425ºF (220ºC) and prepare parchment-lined baking sheet.
In a large mixing bowl whisk together flour, baking powder, baking soda and salt. Grate butter and add it to the dry ingredients, using a butter knife to stir the pieces evenly throughout.
Add cheddar, Parmesan and scallions and stir until evenly combined. Pour in buttermilk and continue to stir until the buttermilk has been absorbed and you can’t see any more flour bits. The dough will seem dry, and that’s okay!
Drop by ¼ cupful’s onto prepared baking sheet, spacing them apart so you get 4 rows of 3 biscuits each. I like to use a butter knife to work around the edges of the measuring cup to pull the dough out onto the baking sheet.
Bake until the tops are golden, about 13-15 minutes. Transfer to a wire cooling rack and let cool for 5 minutes. Serve warm.