To make the mustard butter, beat the butter, lemon juice and mustard to a smooth paste. Store in a jar in the refrigerator.
Mix the chicken and some mayonnaise with a fork, and season with salt, pepper and hot pepper sauce.
Spread half the slices of bread with the mustard butter. Lay slices of lamb or tongue on the bread and spread with the chicken mixture. Add mustard and cress. Top with the remaining slices of bread to make sandwiches, trim the crusts and cut into dainty squares.