Mix together all the ingredients except the sweetbreads and butter, mashing and beating with a fork until thoroughly homogeneous. Stir in the sweetbreads delicately, to avoid crushing them.
Butter a 3-pint charlotte mold (or any mold of simple form and straight sides), press a buttered round of kitchen parchment paper into the bottom, and fill with the mixture, tapping the bottom of the mold two or three times against the tabletop to settle the contents. Poach in a bain-marie, the mold immersed by two thirds in hot but not boiling water in a 350° oven for about 40 minutes. Run the blade of a knife all around the sides of the mold before unmolding onto a heated serving dish. Lift the round of paper from the surface, loosening it gently and progressively with the blade of a knife if it sticks.