How can I eat or drink chocolate as Montezuma would have consumed it in pre-Columbian Mexico?
Chocolate recipe was different for rituals or medicinal purposes. The recipes talk about adding some of the following: vanilla, powdered chilli, flowers as gueyncaztle, mecasuchil, maize as filler, honey, tlacoxoshitl..

Here is the recipe for Mexican hot chocolate from Food and Feasts with the Aztecs, Imogene Dawson (p. 29). It is adapted for modern kitchens:


  Mexican hot chocolate Ingredients 1/2 lb semisweet cooking chocolate 4
  cups milk 1/4 teaspoon ground cinnamon 2 drops vanilla
  1. Break the chocolate into small pieces. Put the pieces in the top of a double boiler or into the heatproof bowl.
  
  
  Fill the bottom of the double boiler or a large saucepan with cold water. Then bring the water to a boil. Turn the heat down so that the
  water continues to boil gently.
  Put the container with the chocolate over the one with the boiling water. With a wooden spoon, stir the chocolate until it has melted.
  Measure out the milk and pour it into another saucepan. Heat the milk gently but do not let it boil. Pour the melted chocolate into the
  hot milk.
  Add the cinnamon and the vanilla to the mixture and bring the mixture to a boil.
  Turn the heat down and whisk the mixture for 2 minutes until it is foaming.
  Pour the chocolate into mugs and use the small whisk to whisk the chocolate again, so that there is foam on the top of each mug.
  


Montezuma dined with Cortes and his Spanish Officers. At this meal the Mexica/Aztec king reportedly drank chocolate from cups of pure gold. Hernán Cortés then wrote: 


  These seeds which are called almonds or cacao are ground and made
  into powder, and other small seeds are ground, and this powder is put
  into certain basins with a point... and then they put water on it and
  mix it with a spoon. And after having mixed it very well, they change
  it from one basin to another, so that a foam is raised which they put
  in a vessel made for the purpose. And when they wish to drink it, they
  mix it with certain small spoons of gold or silver or wood, and drink
  it, and drinking it one must open one's mouth, because being foam one
  must give it room to subside, and go down bit by bit. This drink is
  the healthiest thing, and the greatest sustenance of anything you
  could drink in the world, because he who drinks a cup of this liquid,
  no matter how far he walks, can go a whole day without eating anything
  else"


Source: http://www.foodtimeline.org/foodmaya.html#hotchocolate

I recommend you to read Chapter 8 "From Bean to Beverage", page 100 on the book–180 on the PDF– of History, Culture and Heritage, Louis Evan Grivetti &amp; Howard-Yana Shapiro