Make the dough from the flour and the eggs yolks, put it in a bowl, cover it with plastic wrap, and let it sit.
While you're kneading the dough, bring water to a boil, and boil the sausage and the brains for a few minutes, then remove them and let them cool. Remove the sausage casing and crumble the sausage. Pick over the brain, removing membranes, and mince it.
Blanch the escarole, drain it, squeezing well to extract moisture, and mince it. Sauté escarole until done in the butter. Mince the cooked meats quite finely (if need be you can blend them, but be careful not to make a paste).
Combine all the ingredients of the filling in a bowl and mix them thoroughly with your fingers to obtain a homogenous mixture, seasoning it to taste with salt, pepper and nutmeg.
Take the pasta dough and roll it out dime-thin. Cut the sheet into two equal-sized pieces, dust one lightly with corn meal, roll it up, and cover it so it stays moist. Dot the half of the sheet still on your work surface with blobs of filling about the size of a small hazelnut, putting the dots in rows that are about an inch apart (I have seen agnolotti that are smaller).
Unroll the other sheet, shake off the corn meal, and lay it over the first. Tamp the sheet down well around the filling, so it sticks, and cut the agnolotti free with a serrated pasta wheel.
To cook them, heat the butter in a pan with the sage, and remove it from the flames when the sage begins to whisper (you don't want to brown the butter or burn the sage). In the meantime, bring broth to a boil.
Boil the agnolotti in meat jus, skimming them off, into a serving bowl, as soon as they rise to the surface. Season them with the butter and sage, and serve with freshly grated cheese, and thinly sliced truffle, if you have it.