For the herb butter, combine all the butter ingredients in a mixer fitted with a paddle attachment. Beat until mixed, but do not whip.

For the sandwiches, have the store slice the bread lengthwise on a meat slicer into 1/4-inch-thick slices. (If that isn't possible, you can slice it crosswise with a very sharp knife.)

Lay out 40 slices of bread and spread them all with a thin layer of herb butter. Place smoked salmon on 20 of the slices. Top with the other 20 slices of bread, butter side down. Place the sandwiches on a baking sheet and wrap with plastic. Refrigerate until the butter is very cold. Place the sandwiches on a cutting board. With a very sharp knife, cut off the crusts, cut each large sandwich in half crosswise, and then cut each half diagonally twice to make a total of 40 small triangles. (If the bread was cut crosswise, follow the assembly directions, then cut off the crusts and cut diagonally, twice, to make 20 small triangles.) Serve chilled.