Peel the sweet potatoes and puree them in a food processor. Transfer 3 1/3 cups of the puree to a large bowl. Lightly beat in the flour and egg yolks and season with salt and pepper. Spoon the dough into a pastry bag fitted with a 1/2-inch round tip. Working in 3 batches over a large saucepan of simmering water, pipe out the dough, cutting it into 3/4-inch lengths. Cook the gnocchi for 45 seconds. Using a slotted spoon, transfer the gnocchi to a baking sheet.