Preheat the oven to 180C/160c Fan/ Gas 4. Grease and line 2 x 20cm sandwich cake tins.
Place the chocolate in a heatproof bowl and set it over a pan of boiling water (making sure the bottom of the bowl is not touching the water). Gently melt the particular chocolate while stirring. Once melted, set aside and allow in order to cool at room temperature.
Into a separate bowl, sift the flour, baking powder and bicarbonate of soda together, then add the salt. Set aside.
Cream the butter and sugar together with an electric whisk until pale and fluffy. Continue to whisk while gradually adding the beaten eggs, a little at a time. If the mixture starts to curdle, add 1–2 teaspoons the sifted flour from the other bowl.
Once the eggs are incorporated, whisk in the melted chocolate and vanilla, then about half of the milk, adding it gradually. Add half of the flour mixture plus fold in with large metal spoon, then gradually include the remaining milk and fold in the remaining flour.
Divide between the 2 prepared tins and bake for approximately 30-35 minutes or until the sponges are light and springy, have slightly shrunk away from the sides from the tins and a toothpick inserted into the centre comes out clean.
Allow the cakes to cool for 15 minutes in the tins, then loosen around the edges with a knife and turn onto wire racks to completely cool before icing.
To make the ganache, finely chop the chocolate and put this in a bowl.
Heat the cream in a small pan until steaming. Just before it comes to the particular boil, remove from the heat and pour it over the chocolate. Let it sit undisturbed for 30 seconds, after that start stirring slowly from the middle, working your way outwards until you have a smooth ganache. If there are still a few lumps that aren’t melted, place the bowl over a pan of simmering water and stir until everything is dissolved and smooth. Leave the ganache to cool at room temperature.
Once the ganache is cooled plus thick, spread half of it over one of the cooled sponges. Layer the other sponge on top and finish with the remaining ganache.