Step 1

Preheat to 425°. 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. Immediately slice open lengthwise and scoop out flesh; set skins aside. Pass potato flesh through ricer onto a clean work surface. Let cool.

Step 2

Meanwhile, bring clams, reserved potato skins, and 1 cup water to a boil in a large pot. Cover and cook, shaking pot occasionally, until clams open, 5–8 minutes; discard any clams that do not open. Pluck clams from shells and place in a medium bowl. Cover and chill until ready to use. Pour clam cooking liquid through a fine-mesh sieve into a small bowl. Cover and chill until ready to use.

Step 3

Dust riced potatoes with ¾ cup flour and sprinkle with 2 tsp. or 1½ tsp. salt. Drizzle egg over, then use a fork or your fingers to gently mix ingredients until just incorporated and a smooth dough forms (be careful not to overwork or cooked gnocchi will be gluey and tough), about 3 minutes. Transfer dough to a pastry bag fitted with ½" round tip. (Alternatively, use a large resealable plastic bag and snip off 1 corner.)

Step 4

Pipe dough onto a lightly floured surface into 24" lengths about ½" thick. Cut crosswise into ½" pieces and dust lightly with flour. Arrange gnocchi in a single layer on a lightly floured rimmed baking sheet.

Step 5

Working in 3 batches, cook gnocchi in a large pot of simmering lightly salted water until doubled in size and they float to the top, about 3 minutes (a minute or so more if frozen). Using a slotted spoon, transfer gnocchi to a rimmed baking sheet as they are done cooking. Reserve 1 cup pasta cooking liquid.

Step 6

Melt a third of the butter in a large nonstick skillet over medium-high. Add one-third of gnocchi to skillet and arrange in a single layer. Cook, undisturbed, until dark brown and crisp underneath, about 5 minutes. Transfer to another rimmed baking sheet. Working in 2 batches, repeat with remaining butter and remaining gnocchi.

Step 7

Wipe out skillet and heat oil in skillet over medium-high. Add garlic and cook, stirring occasionally, until golden, about 2 minutes. Add clam cooking liquid and ½ cup pasta cooking liquid and bring to a simmer; cook, stirring occasionally, until sauce is slightly thickened, about 5 minutes.

Step 8

Add clams, gnocchi, and lemon juice to sauce; season with salt and pepper. Cook, gently tossing and adding more pasta cooking liquid as needed, until sauce coats gnocchi.

Step 9

Divide gnocchi among bowls and top with chives.

Step 10

Do Ahead: Gnocchi can be formed 1 month ahead. Freeze on baking sheet until solid; transfer to resealable plastic bags. Clams can be cooked 6 hours ahead. Cover and chill clams and cooking liquid separately.