Heat the oven to 180ºC (160ºC for a fan oven). In the bowl of a stand mixer, beat the butter and sugar together, then gradually add the beaten eggs and vanilla essence. Add the flour, baking powder and cocoa, and mix until everything is combined. Add the melted chocolate and mix again until the mixture is smooth.
Pour the mixture into a lined square baking tin, spread evenly and bake on the middle shelf of the warm oven for around 22–25 minutes until cooked through. Remove the cake from the tin and turn it out onto a wire cooling rack - leave to cool completely.
To make the ganache, finely chop the two types of chocolate and put them in a saucepan with the double cream. Put the pan over a low heat to melt the mixture, stirring continuously until it is smooth. Pour the ganache into a shallow dish to cool, then put it in the fridge to set.
To make the “soil”, boil about 100g of granulated sugar with a few teaspoons of water. Allow the very hot syrup to boil without stirring for a few minutes until it is thick and sticky. Melt 100g of dark chocolate in the microwave or in a bain-marie (a metal bowl over a pan of boiling water), then add the melted chocolate to the sugar syrup and stir until it hardens to form "chocolate soil"!
Be careful as the sugar is very hot - this is not suitable for children to help with this part.
Cover the chocolate cake with the ganache after it has cooled, then add the chocolate “soil” on top.
Finally, add a seed such as sesame, pumpkin, poppy seeds, or even green colored crumbs and green leaves to bring your garden to life.
A recipe originally appeared as part of the Prynhawn Da programme.