Heat a large dutch oven over medium heat. Toast mustard seeds, cinnamon, black peppercorns, cloves, allspice, coriander, juniper berries, and bay leaves until fragrant, about 30 seconds.
Add water, pink salt, kosher salt, and brown sugar to the pot and bring to a boil. Reduce heat to a simmer and stir until the sugar and salts have dissolved.
Remove the dutch oven from the heat and stir in the ice cubes. Chill the brine down until it reaches 45°F/7°C.
Transfer the cooled brine to a large 12-quart plastic container. Submerge beef brisket into the brine and place a plate on top to weigh it down.
Place in the refrigerator for 7 days, checking daily to make sure the corned beef is submerged in the brine.
Remove corned beef from the brine, rinse, and pat dry. Now you are ready to cook your corned beef.