Method

1. Preheat the oven to 180C/350F/Gas 4.

2. To make the cake, grease and line a 20x30cm/8x12in rectangular cake tin with baking paper.

3. Beat together the butter and sugar with a hand-held mixer until pale and fluffy. Add the eggs, a little at a time, beating well between each addition. Add the vanilla extract and the black gel food colouring, if using, with the last of the egg.

4. Sift together the cocoa powder and flour into a bowl. Fold the flour into the butter mixture until fully combined. Spoon the batter into the prepared tin and smooth the surface with the back of a spoon.

5. Bake for 30 minutes, or until springy to the touch (a skewer inserted into the centre of the cake should come out clean). Leave to cool for 10 minutes in the tin before turning out onto a wire rack to cool completely.

6. To make the chocolate ganache icing, put the cream into a saucepan and bring just to the boil. Pour into a heatproof bowl. Add the chopped chocolate, a little at a time, and stir until completely melted and the ganache is smooth and glossy. Leave to cool for a few minutes until it feels just warm to the touch.

7. Place the cake (on the cooling rack) onto a large sheet of newspaper or baking paper. Pour half the ganache onto the centre of the cake and spread out evenly to coat the top and sides, adding more as needed, to make a nice even coat. Allow to cool and set.

8. Meanwhile, lightly butter a non-stick saucepan, add the marshmallows and melt over a low heat, stirring frequently (or place in a lightly buttered heatproof bowl and microwave in 10 second bursts until melted, stiring often). The marshmallows don’t need to melt completely before you remove them from the heat – they will continue melting on their own – but don’t burn or overheat the marshmallow.

9. Check the marshmallow isn’t too hot before taking a pinch between your fingers and pulling it appart so it makes thin strands. Drape the strands all over the cake in an irregular zig zag fashion. When the marshmallow in your fingers cools too much to pull, rinse your fingers in warm water and start again with another pinch. Place the edible eyes in pairs in the gaps between the marshmallow cobweb.