Why apple butter (not apple juice)
Apple butter is slow-cooked, spiced, and concentrated, so it delivers deep baked-apple flavor without watering the drink down the way apple juice does. It also adds a little body that helps the foam. If you only have apple cider, reduce it on the stove first to thicken and concentrate it, or add a bar spoon of apple preserves.
What the egg does, and is it safe?
The whole egg is what makes this a flip. Shaken hard, it turns the drink creamy and gives that custard-like top, and it rounds off the sweetness so the cocktail tastes rich rather than sugary. On safety: the risk from a fresh, properly stored egg is very low, but if you are serving someone pregnant, elderly, or immunocompromised, use a pasteurized egg. For an egg-free version, swap in three-quarters of an ounce of aquafaba (chickpea liquid), which foams almost identically.
The dry shake (do not skip it)
Always shake a flip without ice first. The dry shake lets the egg emulsify and build foam before the ice dilutes and chills everything. Shake it longer than feels necessary, add ice, then shake again to get it cold. Skipping the dry shake is the number one reason a homemade flip comes out thin instead of velvety.
Make it for a crowd
For a party, batch the anejo, apple butter, and caramel in a jar at the same ratio and keep it cold, but add the eggs and shake to order in small batches, since egg foam does not hold if it sits. Two servings at a time in a large shaker is the sweet spot.
Non-alcoholic caramel apple version
Leave out the tequila, keep the apple butter, caramel, and egg, and add a splash of cold brew or apple cider for backbone. You still get the caramel apple flavor and the creamy flip texture without the alcohol.