Preheat oven to 18º0 C. Grease a loaf tin with some butter and dust with some flour.
Cream together butter and castor sugar in a bowl with an electric beater. Add eggs gradually and whisk well. Add vanilla essence and whisk.
Sieve together flour and baking powder into the mixture and fold in well. Divide the batter equally into 2 different bowls.
Take cocoa powder in a bowl. Add oil and mix well.
Add cocoa-oil mixture to one portion and mix well. Add coffee decoction to the other portion and mix well.
Pour ½ of coffee batter into the loaf tin and put cocoa batter on top. Put remaining coffee batter on top, insert a fork and swirl to create marble effect, level it out and tap.
Place the tin on a baking tray, place the tray in the preheated oven and bake for 25-30 minutes. Remove from oven, cool and demould.
Dust with icing sugar and slice. Serve.