Preheat oven to 180 degrees C. Grease and flour one 13x23cm cake tin.
In a large mixing bowl, cream butter and sugar until light. Beat in eggs, one at a time; add 1 teaspoon vanilla.
Sift together flour, baking powder, bicarb soda, 1/4 teaspoon salt and cocoa powder. Add to creamed mixture alternately with water, beating after each addition. Stir in sauerkraut. Pour batter into prepared pan.
Bake at 180 degrees C for 35 to 40 minutes. Let cake cool in pan. Frost with sour cream chocolate icing. Cut into squares to serve.
To make sour cream chocolate icing: Melt the semi-sweet chocolate pieces and 60g butter over low heat. Remove from heat. Blend in the sour cream, 1 teaspoon vanilla and 1/4 teaspoon salt. Gradually add sifted icing sugar to make spreading consistency. Beat well. Spread over cooled cake.