Add the dry ingredients to a bowl and use a whisk to mix together. Make sure there are no lumps.
Cut the butter into 1 cm cubes and cool. You can also use a chilled spoon to reduce their size before mixing them in.
Mix the butter in using a spatula in a cutting motion. Use your hands at the end to knead into a crumbly texture.
Make a depression in the center and add the whey (or milk) in 2-3 parts. Let it soak in without kneading. Gather it together in one lump and place in a plastic bag.
Stretch it out to a 1 cm thinness while still in the bag (it should be 15 x 20 cm large). Using a rolling pin helps things go more quickly. Let sit in the refrigerator for 1-2 hours.
Preheat the oven to 200. Remove the dough from the bag, and fold two or three times. Press down on it to get it to stick together.
Use a knife or a cookie cutter to make the individual biscuits. Brush milk on top, place on a baking pan lined with parchment paper, and bake at 200 for 18 minutes.
They're delicious hot from the oven! If they cool down too much, you can warm them up a bit and enjoy with butter or jam.