Preheat your oven to 220C/420F.
Add whole wheat flour, white flour, baking powder, soda and salt to a large mixing bowl. Mix together with a spoon.
Add the cubed butter to the bowl. Using a knife or a pastry cutter, cut the butter into the flour until the butter pieces are the size of peas.
Add the discard and the milk to the flour and butter mixture. Mix it in with a wooden spoon or a dough whisk until it comes together into a ball. You may need to use your hands towards the end of mixing, but try to do most with the spoon (your hands will warm the butter too much).
Tip the rough ball of dough onto your counter (but don't flour the counter yet). Knead VERY briefly until it's smooth. Don't overhandle the dough (you want the butter to remain cold).
Lightly flour your counter and roll out the dough into a 1 inch circle. Using a round cookie cutter or something similar (I used the rim of an old thermos flask) cut out the biscuits and place them onto an ungreased baking tray.
Bake on the middle shelf of your oven for 10-12 minutes until risen and golden brown on top.
Let the biscuits cool a little on cookie racks before serving. You can cut them in half and spread with some butter or serve them alongside a soup or stew, or even with some scrambled eggs for breakfast. They are very versatile!