1. Preheat the oven to 180C/350F/gas 4. Butter and flour or line with parchment a round 23cm (9in) cake tin.
2. Put the chocolate and cocoa in a bowl, pour over boiling water, stir to melt and cool down. In the meantime sift the flour, baking powder, soda and salt into a bowl.
3. Beat the butter with the sugar in a standing mixer or with a hand-held one, until fluffy. It might look a bit grainy, but when you add the eggs, one at a time, mixing constantly, it will look better. Add the cooled down chocolate mix and beat in. Add a third of flour, then half of the milk, flour and milk again, ending with the rest of the flour. Beat until combined.
4. Pour the mix into the tin and bake for about 50 minutes, until the skewer inserted in the middle comes out clean. Cool in the tin for about 10 mins, then turn out upside down onto a greased cake rack. If you used parchment, don’t peel it off until completely cold.
5. The cake must be cold to be cut; it’s a good idea to put it in the fridge for half an hour before cutting. Using a good bread knife, mark the line of the outside of the cake halfway down its height (although no matter what you do, the top layer will always come out slightly higher). Slice through; leave the bottom half on the plate you are going to keep it on, and slide the other half off using a bread peel or a palette knife.
6. Heat up about 3 tbsp. of apricot jam in a pan with a tablespoon of water, then pass it through a sieve. Spread the jam over the bottom half of the cake smoothing the crumbs. Spread some also over what will be the top of the cake.
7. Place the milk and dark chocolate for the ganache in separate bowls, breaking them into little pieces. Put the cream and butter in a pan and bring to a boil. When it boils vigorously, pour 80g over the milk chocolate and the rest over the dark chocolate (you want the inside filling to be a bit thicker so it doesn’t spill over the sides). Leave to rest for a couple of minutes, then whip each lightly with a whisk. Leave for half an hour or so to thicken a bit.
8. Pour or spoon the milk chocolate ganache over the apricot jam covered bottom half of the cake and spread evenly – not too much as it will find its space once sandwiched. Put the top back on, cut side to the ganache. Now pour the dark chocolate ganache over the top of the cake and smooth with a palette knife. The butter makes it shiny; the sides can be done by scraping ganache off the base and plastering to the sides.