Cream together butter and sugar with electric mixer at hi speed, until light and fluffy. Add eggs, one at a time, beating well after each. Sift together dry ingredients. Mix coffee and vanilla. Add dry mix and coffee to butter, alternately, beginning and ending with dry mix. Mix by hand using spatula or wooden spoon. Do not overbeat. Remove one third of the batter and mix with melted chocolate. Spread remaining two third batter in greased and floured tube pan. Put clumps of chocolate batter on top of other batter and swirl with knife. Bake 1 hour at 180ºC Cool 10 minutes then turn onto plate. Cool completely.