Combine the egg, mustard, mayonnaise, chives and pepper in bowl. Lightly butter one side of each bread slice. Divide and spread half the egg mixture on the buttered side of three of those bread slices. Set 2 to 3 slices (depending on width) of smoked salmon on each sandwich. Divide and spread the remaining egg mixture on the smoked salmon. Set on remaining bread slices, buttered side-down. Wrap and refrigerate sandwiches at least an hour before serving. When ready to serve, trim away crusts and cut each sandwich into 4 bars, squares or triangles, set on a serving platter and enjoy.
Note: No salt is added to this recipe because the mustard and salmon add saltiness.