Combine the apple cider vinegar with almond milk. Stir, let sit for 5 minutes to curdle.
In the meantime, pour flour mix into a large bowl and stir in the sea salt. Add in the butter and use the pastry cutter (or 2 knives) to chop up the butter until it resembles small, pea-sized crumbs.
Pour in the beaten egg and the milk and vinegar mixture. Stir until all is combined and the dough becomes slightly sticky with a few crumbs. Add 1-2 extra tablespoons of milk, as needed.
Next, break off about a 1/2 cup size amount of dough and roll between palms, forming into a ball. Repeat until you have about 7-8 equally sized dough balls. Place them spaced out on a plate and gently press down on each biscuit, flattening it a little on top. Cover the plate and chill in the freezer for 10 minutes.
Preheat the oven to 425°F. If using an egg wash, mix the egg yolk and tablespoon of water in a small bowl and set aside.
Remove the plate from the freezer and space out the biscuits 1/2'' apart or more on a cast iron skillet or a baking sheet that can withstand the heat. Lightly brush the egg wash over the biscuits. Place the skillet on the middle oven rack.
Bake the biscuits for 20-22 minutes or until lightly golden and an inserted toothpick comes out clean.
Allow the biscuits to cool for 5 minutes and then use a spatula to transfer the biscuits onto a cooling rack. Serve warm.