I think cacao is best eaten (nearly) pure - cacao, a little sugar, cocoa butter if necessary, and nothing else. There a rare exceptions; one is heavy cream. Whisking dark chocolate into hot cream makes a sublime cupful of chocolate and dairy fat bliss, and it's extremely simple. For one serving, heat 1/4 heavy cream in the microwave until just bubbling, about 30-40 seconds. Add 2-3 tablespoons of chopped chocolate. Let sit for a moment so the chocolate melts, then whisk until smooth. The little electric milk frothers work great. Pour into an espresso-size cup and serve.
Sixty-five to 75% cacao works well for classic dark, use stronger if you like it intense, or try dark milk for richer lactic flavors. Pacari's Chili bar (lovely) also made great spicy drinking chocolate - chili is an occasional exception to the pure chocolate stance.
If you're out and about in Portland, OR, be sure to stop by Creo (bottom left) or Cacao (bottom right) for their drinking chocolate.
