Place the chopped eggs, mayonnaise, sour cream, and salt and pepper in a bowl. Mix well, mashing the egg as you go so that the egg yolks become part of the sauce. Set aside.

Spread a tablespoon of butter on each piece of bread. Arrange the cucumber over the top.

Spoon the egg salad over the cucumber, and give it an extra grind of black pepper. 

Finally, finish with some pieces of pickled herring and a sprinkle of chopped fresh dill.