Pre-heat the oven to 180`c. Grease and line a small loaf tin or 20cm/8in round tin. Cream the butter and sugar with an electric whisk in a large mixing bowl until very pale and fluffy. Add the beaten eggs, a little at a time, whisking well after each addition. Then whisk in the vanilla essence. Sift the flour and salt into a large bowl, then fold it gently into the butter and egg mixture. Divide the cake batter into two equal portions (about 350g each). Add the milk to one portion of the mixture, stirring in gently until well combined. Mix the cocoa powder and the coffee together in a mug to form a smooth paste. Stir this paste into the other portion of the cake batter until thoroughly combined. Spoon large blobs of each cake mixture into the prepared tin, alternating the flavours to give a marble effect. Bake for 50-60 minutes until the cake is springy to the touch and a wooden skewer inserted into the centre of the cake comes out clean. Allow the cake to cool in the tin for ten minutes, then turn out onto a wire rack to cool.