There is a small recipe card here. It's clearly not in Twilight, Pinkie Pie, or Spike's hoofwriting. There is no magic invisible ink. Most of the ink on the card is a deep red.

If you don't have a vampony infection near you, here's a nice recipe to tide you over until we arrive! (It's an acquired taste, but isn't everything, really?)

The Darkhoof

Recipe:
1-1/2 cup red wine
1 teaspoon pure lime juice
1/4 teaspoon fresh ground cinnamon
1/8 teaspoon living blood from a sapient being *

Instructions:
Mix gently. Make sure the blood is added last, and to the center of the concoction (it must not touch the sides of the container before mixing). **

Serves one, but the ingredients can be scaled up. The victim gastronome should ideally drink until they are just barely intoxicated enough to make minor errors in judgment. Enjoy!

( * = I suppose this could be optional, but why not get a head start, darling? )
( ** = If desired, cornstarch may be added in order to turn the drink syrup-like. It's not quite as appetizing, but it's more accurate this way. C'est la vie, nothing quite compares to the real thing, I'm afraid. )

Nothing is written on the reverse side of the card, but it smells faintly of cloves.