Pluck clams from shells and place in a medium bowl.
Cover and cook, shaking pot occasionally, until clams open, 5–8 minutes discard any clams that do not open.
Meanwhile, bring clams, reserved potato skins, and 1 cup water to a boil in a large pot. Pass potato flesh through ricer onto a clean work surface. Immediately slice open lengthwise and scoop out flesh set skins aside. Prick potatoes all over with a fork and place directly on oven rack roast until flesh is very soft when squeezed gently and skin is crisp, 60–75 minutes.
