Butter and line one 20cm round springform cake tin or two loaf tins. Preheat oven to 160 C / Gas 3.
Break dark chocolate into a pan, combine with butter, instant coffee and water and heat on a low heat until all melted.
In a bowl, combine flour, bicarbonate of soda, sugar, cocoa powder and beaten eggs. Mix in melted chocolate mixture slowly to avoid lumps. The resulting mixture will be much runnier than usual sponge mix.
Pour into tin(s) and bake on middle shelf of oven for about 30 to 45 minutes, or until the cake is still very gooey inside but with a thick crust on top.
In another bowl, roughly break up the chocolate flakes, white and milk chocolate into largish lumps of about 1.5 to 2cm (or to size desired). Remove the cake(s) from the oven and arrange the broken chocolate across the top by pushing the pieces into the crust, leaving them protruding. Then return to the oven for another 30 to 45 minutes, or until a skewer comes out clean.
Remove cakes from oven and allow to cool before removing from the tin. Store in an air-tight container in a cool dry place.
No need to use expensive or cooking chocolate either for in the cake or on top, your supermarket own brand should suffice.
The timings are a bit rough as my oven is very temperamental about baking, so be careful to check progress often the first time you try this recipe.