Cook the sweet potatoes, whole and unpeeled, in very little water for 30 minutes to steam. Once cooked, peel and mash them well or put through a potato ricer.
Mix the flour into the sweet potato mash, season with sea salt and freshly ground black pepper. Tip the dough onto a lightly floured surface and knead lightly until combined. Shape the dough into three or four balls.
Dust the surface with more flour if necessary. Using your fingertips, roll the dough into a sausage about 2cm in diameter. Cut the dough into 2.5 cm pieces, then roll the dumplings against the front of a fork to create ridges (this will help hold the sauce once cooked.)
For the sage butter, place a frying pan over a medium heat and add the butter. When the butter has melted add the sage and cook until it is slightly crispy
Tip the potato dumplings into a frying pan and mix them gently in sage butter sauce and cook for 5 minutes, turning to brown each side.