Step 1

Toast chiles, ginger, cinnamon stick, cardamom, bay leaves, peppercorns, coriander seeds, mustard seeds, allspice berries, juniper berries, and cloves in a large dry skillet over medium heat, shaking pan occasionally, until spices start to jump in pan and are fragrant, 3–5 minutes (don’t let them burn). Transfer to a mortar and pestle and coarsely crush about 1 minute (or crush with a knife).

Step 2

Whisk Guinness, brown sugar, curing salt, 2½ cups kosher salt, and 1 gallon water in a large pot or container (at least 8 quarts) until sugar and salts are dissolved. Scoop out ½ cup pickling spice and set aside in a small bowl or jar (you won’t need it again until it’s time to cook the corned beef). Add remaining spice mixture and brisket to brine and weigh down with a plate to keep it submerged (it’s okay if brisket needs to be crammed into the container, just don’t fold it; you can also cut it in half if you can’t find a large enough container). Cover with plastic wrap or a lid and chill, flipping meat once a day, 5–6 days.

Step 3

Bring a large pot of lightly salted water to a simmer. Remove brisket from brine and rinse thoroughly under cold water; discard brine. Add brisket to simmering water along with reserved ½ cup spice mixture and cook, covered, until corned beef is fork-tender and easily shreds with the grain, 4–4½ hours.

Step 4

Slice corned beef against the grain and serve.Do Ahead: Corned beef can be cooked 1 week ahead; cover with plastic and chill.