Place a small pot of water on to boil, turn down to a simmer.
Place the 300 g of chocolate in a heat-proof bowl, sit it on top of the hot water and stir occasionally until you obtain a smooth, glossy melted chocolate.
Remove the melted chocolate from the hot water and spread evenly with a palette knife across a strip of clean acetate.
Shape the acetate into a cylinder and place on a tray in the refrigerator to set.
Soften the gelatine in a bowl of cold water to soften.
Place the 120g chocolate in a heat proof bowl
Place the cream and milk in a saucepan, add the cocoa powder and bring to a gentle simmer, stirring well to break up the cocoa powder.
When simmering, turn off the heat and add the squeezed dry, softened gelatine.
Whisk to dissolve,then pour over the chocolate and let it stand.
After 2-3 minutes, slowly stir the cream through the chocolate until smooth.
Cool for 20-30 minutes, by this time the cream should have slightly thickened, pour into the moulds taking care that they don't leak too much.
Place in the fridge to set to a firm-ish jelly-like texture.
When firm, gently peel off the acetate sheet, turning the panna cotta on to the plate.
«If the cream mix is still to liquid, pour in just a little to fill the base of each mould and set in the fridge for 5 minutes. This will be enough to set a base and waterproof the moulds.»