Roughly chop the peanuts. Slightly crush the cornflakes.
Mix the dry ingredients together.
Melt the butter and mix in the golden syrup and vanilla essence.
Add the wet ingredients to the dry ingredients and mix.
Press into a rectangular or square tart tin that has been greased with butter.
Bake at 180ºC (350ºF) for 25 - 30 minutes until golden brown.
Leave to cool in the tin before icing the slice with chocolate icing.
For the Icing:
Add the cocoa powder and butter to the boiled water and stir until dissolved. If they butter doesn't melt completely, put the mixture in the microwave for a few seconds or on the stove-top.
Add the icing sugar and whisk until you have a smooth, runny mixture.
Pour onto the biscuit base and leave to set.
When set, you can cut the slice into squares or bars. It keeps well for 2 -3 weeks in a closed container.