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1
Place clams in a large bowl of heavily salted cold water. Let soak 20 minutes. Drain and scrub any clams that are still dirty or sandy.
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2
Pulse butter, miso, ginger, lime zest, and red pepper flakes in a food processor until smooth and well incorporated. Set aside half of miso butter for serving. Transfer remaining miso butter to a small bowl.
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3
Prepare a grill for medium-high heat. Arrange 2 layers of 2 large pieces of foil on a work surface, overlapping to create one large surface area. Place potatoes in center of foil in 1 layer and dot with 2 Tbsp. miso butter. Season with 1 3/4 tsp. salt. Fold up sides of foil, then crimp and fold to seal. Transfer to a large rimmed baking sheet.
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4
Arrange 2 more layers of 2 large pieces of foil in the same way to create 1 large surface area. Place half of clams in center of foil, dot with 1 1/2 Tbsp. miso butter, and enclose in a purse shape (so you can take a peek at clams when cooking). Repeat with more foil and remaining clams. Transfer both packets of clams to pan with potatoes.
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5
Arrange shrimp, corn, and scallions on another large rimmed baking sheet in separate sections. Drizzle with oil; season with 1 1/4 tsp. salt. Toss to coat.
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6
Grill potatoes, turning every 5 minutes, until tender when pricked with a toothpick, about 25 minutes total. Return potatoes in foil back to baking sheet to keep warm.
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7
After potatoes have been cooking for 10 minutes, grill corn, brushing occasionally with miso butter and turning every 3-4 minutes, until charred in some spots, about 10 minutes total. Return corn to baking sheet and cover with foil to keep warm. Grill clams until they open, 10-15 minutes. Transfer to pan with potatoes and corn.
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8
Grill scallions and shrimp, brushing with miso butter and turning once halfway through, until scallions are charred and shrimp is cooked through, 4-6 minutes, depending on their size. Return to baking sheet.
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9
Cut each ear of corn into 4 pieces and divide between 2 platters. Divide potatoes, shrimp, scallions, and clams between platters, then pour clam juices over. Top with cilantro and sesame seeds. Serve with reserved miso butter, lime wedges, and lots of napkins.
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10
Miso butter can be made 2 days ahead. Cover and chill. Potato packets and clam packets can be grilled 1 day ahead. Enclose packets and chill.
