Steep the porcini for 10 minutes in a small amount of boiling water, then drain them well, reserving the liquid. Blend the porcini. Strain the liquid, which may contain sand, into a pan, add the porcini, and heat, stirring constantly, until the mixture has thickened. Let the mushrooms cool, and combine them with the other ingredients when you make the pasta. This pasta will be delightful with unsalted butter and a dusting of cheese, or a simple cream sauce.