Preheat oven to 180 degrees C. Slightly grease and flour a 25cm /10" cake tin.
Combine butter, peanut butter and both types of sugar in a mixing bowl. Beat until light and creamy, about 3-5 minutes on low. Add eggs one at a time, beating well after each addition.
Sift together the flour, baking powder and salt.
Gradually fold the sifted flour and milk, alternately, into the egg mixture, stirring well after each addition.
Divide mixture into two equal portions. Add chocolate chips to one portion; mix. Blend the coffee granules, essence and cocoa together and mix lightly until the ingredients are well-blended to get a mocha-flavour - add this to the other portion.
Put a ladleful of choc-chip batter into the prepared tin, top this with a ladleful of mocha-flavoured batter and alternate the two steps until both portions are used up.
To get a marble effect, draw a wooden skewer through the batter, taking care not to touch the bottom of the tin. Lightly shake the tin to get rid of any air bubbles.
Bake in a preheated oven for 25 mins. Then turn the oven temp down to 160 degrees C. Continue cooking for another 25 minutes.
To test the cake, touch it lightly with the fingertips. If it is cooked through, the cake should spring back immediately when touched. Cool the cake in the mould for 5 to 10 minutes before turning it out onto a wire rack.
To prepare the glaze, combine apricot jam and water in a small saucepan. Heat gently to melt the jam, stirring continuously.
Brush cake with glaze. Leave to cool before serving.