Set up a double boiler using a large heatproof bowl set over a pan of simmering water. Melt butter, chocolate and sugar. Mix instant coffee and hot water together and add to the mix.
Once all is melted and combined, take off heat and add milk. Whisk in one egg at a time using an electric whisk. Now, whisk in vegetable oil.
Sieve in the flour, cocoa, baking powder, salt and bicarb. Mix well, making sure there are no pockets of flour. Pour into a greased 20cm (8 in) round cake tin.
Bake in a preheated 150 C / Gas 2 oven for 2 hours.