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Mole Poblano!!!
This is the place to get it. There were 2 memorable meals on my last trip, the Mole Poblano at San Angel Inn and the Portobello Mushroom Pizza and Butternut Squash Soup at Wolfgang Pucks.
The atmosphere at San Angel Inn was dark, but nice. Also, if you can, request a table by the back of the restaurant near the water and volcano, it was a fantastic view that added to the experience.