To make the sweet potato gnocchi, place the sweet potatoes on a baking tray and prick all over with a fork. Bake the sweet potatoes with their skins on in the oven at 200`c for 35-40 minutes, depending on their size. When they are ready, remove them from the oven and allow them to cool for 5 minutes. Next, slit the potatoes down the middle and scoop out the cooked flesh, discarding the skins. You should be left with about 450g of sweet potato. Add in the egg yolk and cinnamon and season well with salt and pepper. Mix the ingredients together well with a spoon before adding in the flour. Quickly work the flour into the sweet potato mixture to produce a nice, smooth gnocchi dough. Turn the dough out onto a lightly floured work surface and knead lightly for a minute. Cut the dough into three or four pieces and roll each piece into a long, tubular shape, about the thickness of your finger. Cut off pieces 2.5cm/1in long using a sharp knife. You can leave the gnocchi in this shape. As you go along, place the gnocchi onto a tray sprinkled with the semolina, and also sprinkle some on top of the gnocchi to avoid them from sticking together. You can now prepare the sauce for the gnocchi by heating the olive oil and butter in a large pan. Add in the garlic and thyme and cook for 2 minutes. Add the diced pumpkin to the pan, season well with salt and pepper and continue to cook for about 5-10 minutes, or until the pumpkin is cooked but still firm. Add the mascarpone cheese and white wine to the pan, lower the heat and allow the sauce to reduce and thicken for 2-3 minutes. Add in ¾ of the chopped walnuts and turn off the heat. You can now cook the gnocchi by dropping batches into a large saucepan of salted boiling water, taking care not to overcrowd the pan as the gnocchi will stick together. As soon as the gnocchi are cooked, they will float to the top of the water (this should take only 2 or 3 minutes). Remove the cooked gnocchi from the pan with a slotted spoon and add to the pan with the pumpkin. Gently toss the gnocchi in the juices to coat all over. Spoon the sweet potato gnocchi immediately into deep pasta bowls and sprinkle over the remaining chopped walnuts before serving.