If baking biscuits by themselves (not on top of a casserole): Preheat oven to 400 degrees Fahrenheit. Line a cookie sheet with parchment paper. Set aside. (If using biscuits to top a casserole, follow casserole instructions for baking details.)
Combine cold water (or cold coconut milk) and apple cider vinegar in small dish. Set aside.
In large bowl whisk together dry ingredients: cassava flour, collagen, sea salt and baking soda. Set aside.
Cut cold fat into flour using food processor, (or 2 knives). Pulse to combine until largest fat pieces are pea-size.
Pour water (or milk) and ACV into flour mixture, and stir to just combine. Do not over-mix. (I use no more than 15 strokes with my rubber spatula.)
Dump dough out onto parchment lined cookie sheet. Form into a 2 inch thick rough square. Do not pat it down. (For casserole, dumping out dough is optional; a cookie scoop can also be used. See next step for details.)
Using a sharp knife, cut straight down into dough. Dip knife in flour after each cut, until you have all biscuits cut. Spread out biscuits just slightly from each other, so they have room to cook and expand slightly. (For casserole, use either cut biscuits, or use large cookie scoop to portion dough. Spread separate biscuits evenly out over casserole surface. Follow baking instructions from casserole recipe.)
Bake in preheated oven 25-30 minutes, until puffed, golden and cooked through.
Enjoy! Split open and top with either sweets or savories!