Spread half the block evenly over one cocoa bread sheet. Sprinkle with the walnuts. Add the smoked duck breast slices, forming an even layer edge-to-edge.
Spread the next sheet with the black cherry jam. Sprinkle generously with rocket and place it on the first sheet.
Spread the remaining sheet with the remaining foie gras and lay it upside-down on the others.
Gently press down to bind the sandwich together.
Using a long, narrow serrated knife: cut the sandwich from top to bottom make three long strips, then divide those into four. Cut the 12 resulting squares diagonally. Serve 2 triangles per person, held together with a cocktail stick.
Let your imagination go! You can replace the black cherry jam with fig jam, or even quince jam. You can add Granny Smith apples, thin slices of Basque sheep’s cheese and more.