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For mole sauce:
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In a comal or skillet over MEDIUM-HIGH heat, toast the chiles until they are soft and pliable, about 3 minutes.
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Remove chiles from comal or skillet, and place in a bowl. Cover with hot water and allow to steep for 15 minutes.
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On the same comal or skillet, place corn tortillas, toast turning to toast on both sides, and remove tortillas from the comal.
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Reduce heat, and add almonds, peanuts, pepitas, coriander seeds, and cinnamon sticks. Lightly toast for about 2 minutes, stirring often to prevent burning. Remove almonds, peanuts, pepitas, coriander seeds, and cinnamon sticks.
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In a blender or food processor, add the toasted tortillas in pieces, almonds, peanuts, pepitas, coriander seeds, cinnamon sticks, black pepper, garlic, Mexican chocolate, and 2 cups of beef broth.
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Blend until smooth. Add the onion, prunes, hydrated chiles, and a few spoons of steeping water from the chiles. Continue blending, adding more steeping water if needed.
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Heat olive oil in a Dutch oven or stock pot over MEDIUM-HIGH heat.
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Carefully add sauce as it might splatter. Fry sauce, stirring often, for 3 minutes. Add remaining beef broth, brown sugar, and salt. Reduce heat, and simmer for 10 minutes.
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Allow sauce to cool completely; use to braise Brisket.
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For brisket:
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Place Brisket on a large baking sheet. Pat the Brisket dry and season all over with salt and pepper.
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Rub mole sauce over the entire Brisket. Place Brisket uncovered in the fridge overnight.
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Preheat the oven to 350°F.
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In a large roasting pan, add sliced onion and the remaining broth. Place Brisket in the roasting pan fat side up. Cover with foil, and place in the oven for 3 1/2 hours or until Brisket is fork tender.
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Remove the roasting pan from the oven and place Brisket on a cutting board; allow it to rest for 15 minutes.
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Slice Brisket against the grain and serve warm drizzled with mole sauce.