This idea came from a couple of different people, and I finally got around to making these last week.
Essentially the flavours come from the white chocolate ganache, coffee ganache, and the cocoa powder on the top. Perhaps next time I may add a splash of marsala or some other liqueur to the white chocolate ganache to add an extra kick.
For the shells, see the generic macaron recipe post here. Colour the shells with a tablespoon of cocoa powder (remove 1 tablespoon of icing sugar from the shells). Dust the shells with cocoa powder once they are baked and cooled.
For the white chocolate ganache, heat 100mL of cream and pour over 200g of good quality white chocolate. Allow to sit for a few minutes then mix until it becomes a smooth, glossy mixture.
Decant about 1/3 of the white chocolate ganache into a separate bowl. Dissolve 2 tablespoons of instant coffee in the smallest amount of hot water you can to make a paste - add this to the smaller bowl of ganache and mix until combined.
Cover the macaron with white chocolate ganache, and pipe a small circle of coffee ganache in the centre. Sandwich with another macaron and leave in the fridge to mature for at least 24 hours.