For the cake, preheat an oven to 150 degrees/gas 2. Grease and line an eight inch/20cm cake tin with parchment paper.

Put the cocoa, butter, oil and water into a wide saucepan and bring it to the boil. Remove from the heat and then stir in the sugar and flour. Beat together the eggs with the milk and bread soda and then add this mixture to the saucepan. Pour this into the prepared cake tin and bake for 1¼ hours

Allow the cake to cool and then turn it out onto a wire tray and let it get completely cold, which will take a few hours.

Meanwhile make the Swiss meringue. Place the egg whites and sugar in a large metal or glass bowl. Set the bowl over a pot of simmering water, and whisk, continuously, until the sugar has completely dissolved and the mixture looks opaque.

Remove the bowl from the heat, add vanilla extract, and whip on high speed until completely cool.

Whip in the butter, a tablespoon at a time, until thick and fluffy. If it begins to look curdled, continue to whip until it comes back together, before adding the remaining butter. Fold in the melted chocolate. If the egg whites and/or the butter are slightly too warm, the buttercream may not thicken properly. This can be easily remedied by placing it in the refrigerator for 20-30 minutes, then whipping again.

To make the meringue mushrooms, pre-heat an oven to 130 degrees/gas ½. Line two baking sheets with parchment paper and set aside. In a clean dry bowl, preferable stainless steel or glass, whisk the egg whites and sugar together at full speed until the mixture forms stiff dry peaks. This will take eight to 10 minutes. Put a round tip about 10mm wide on a piping bag and fill the bag with the meringue mixture. For the stems, press out a tiny bit of meringue onto one of the trays, then pull the bag straight up. Do not worry about making all of the pieces exactly the same. The mushrooms will look more natural if the pieces are different sizes. To pipe the mushroom caps, squeeze out round mounds of meringue onto the other baking tray. Pull the bag off to the side to avoid making peaks on the top. Place the cocoa powder in a fine-mesh sieve and lightly dust it over the stems and caps then blow on the cocoa powder vigorously to blur it and give the mushrooms a realistic look. Bake for one hour and then turn off the oven and allow the meringues to cool in the oven. When cool, the stems are attached to the caps using a little melted chocolate to glue them together. The stems may need to be trimmed so use a sharp knife to cut the tip off each stem to create a flat surface.

To assemble: Divide the cake into three evenly-sized layers. Line an eight-inch/20cm tin with cling film, place one layer of cake in the bottom of the tin, spread a thick layer of the icing over this layer, put the second layer of cake on top and repeat with the icing, and put the final layer on top of this. Cover with the cling film and pop into the fridge for 30 minutes to set.

Remove the cake from the fridge, take it out of the tin and remove the cling film. Cover the cake with a thin coating of icing, then, using a ruffle nozzle, pipe the icing on the sides of the cake.

Once the cake is decorated put it in the fridge for 30 minutes or so to set again. Decorate with the meringue mushrooms. This cake will keep for at least a week.