Preheat oven to 450°. Line a cookie sheet with parchment or non- stick foil.
Combine all dry ingredients in a bowl, or in the bowl of a food processor. Add the pieces of very cold butter to the dry ingredients and pulse until the mixture resembles coarse meal. You want pea-sized chunks of butter to remain, no smaller. Use 2 forks or a pastry blender to blend if you are not using a food processor.
Add milk and vinegar to a 1-cup measure and top off with enough whey (approximately 1⁄2 cup) to make 1 cup of liquid. Add the milk mixture to the dry ingredients and butter mix and mix JUST until combined. The dough should be very wet.
Turn the dough out onto a floured board. Gently pat (do not use a rolling pin) the dough out until it’s about 1⁄2 inch thick. Try to handle the dough as little as possible so it does not toughen up. Once it reaches 1⁄2 inch thick, fold the dough over and pat out and fold over 4 more times. After the last time, gently press the dough out into a 1-inch-thick square.
Using a biscuit cutter (or small glass) about 2 to 3 inches in diam- eter, cut out about 8 biscuits. Gently reform the dough and cut 2 additional biscuits. (Note: The second-cut biscuits will not be as tender as the first ones.)
Place the biscuits on the prepared cookie sheet, forming a square so that they touch each other. Add a small pat of butter and a pinch of salt to the top of each biscuit. If using the lemon zest, sprinkle about 1⁄4 teaspoon on the top of each biscuit.
Bake for about 10 to 12 minutes, until light golden brown on top.
* Biscuits can be made ahead and frozen before baking. If frozen, bake in a preheated oven at 450° for 20 minutes when ready to serve.