Preheat the oven to 180°C. In a large bowl, cream together the butter and sugar. Add in the egg yolk and vanilla, continue to beat together. Then add in the flour and cocoa powder and stir until well combined. At this point divide the mix evenly between two bowls.
To one add the melted chocolate and mix through the dough. To the other add the marmite and again mix well. Divide the mix into golfball size balls, and on a lined tray, flatten slightly.
Lay the cookies out, making sure you know which are the tricks and which are the treats! Bake to the oven for 10-12 minutes, so the cookies are cooked but not colouring. Decorate with any halloween inspired decoration you want to.