2) For the sandwiches, slice the bread lengthwise into 6mm thick slices, or if this is not possible slice it diagonally.
3) Butter 40 slices of bread with a thin layer of herb butter. Place the smoked salmon on 20 of the slices and top with the other 20 slices.
4) Place the sandwiches on a baking sheet, wrap with clingfilm and refrigerate until the butter is very cold.
5) Place the sandwiches on a cutting board and with a very sharp knife, cut off the crusts. Cut each sandwich into 4 diagonally and serve chilled.