Clean the leek removing the roots and the tougher part, wash the rest and with a mezzaluna chop it up finely. Place it in a pan with a dash of oil, then fry it until soft but not golden over a very gentle heat. Add the crumbled tofu, mix well and brown for a few minutes. Mix a spoonful of soy sauce to with the leek mix, ladle the broth and simmer for 20 minutes, over a medium heat, stirring from time to time. As soon as the sauce is ready, add the sesame butter. Cook the pasta in plenty of boiling water, drain al dente, pour it in a soup tureen, fold in the sauce, mix well and serve.