Pre-heat the oven to gas mark 4 (160 C), and line three 7 inch cake tins with grease proof paper.
In a large bowl, whisk together the butter and the sugar until soft.
Beat in the eggs one at a time until and beat well until the mix is creamy and pale.
Add the zest and juice of the orange and mix until combined.
Sift the flour and salt into the bowl and fold through until the flour is fully incorporated.
Divide the cake mix across three bowls and colour each bowl of mix a separate colour.
Pour the cake mix into three separate cake tins and bake for 35 mins until springy to the touch and a skewer comes out clean.
Allow to cool in the tins for 10 minutes before turning out on to a cooling rack.
Once the cakes have cooled completely, melt the chocolate either slowly in the microwave or a bain marie.
Place the chocolate to one side and allow to cool slightly but make sure it stays pourable.
Heat the milk gently and stir in the cocoa powder.
In a large bowl, beat together the butter, icing, cocoa powder and milk.
Slowly add the melted chocolate beating well.
Finally, beat in the orange juice and black food colouring.
Spoon half the mix into a piping bag with a star tip nozzle and leave the rest in the bowl.
Take the cake layer you would like to sit on the bottom and push a circular cookie or pastry cutter halfway through the cake. Scoop out the cake.
Repeat this with the layer you'd like to be the top of the cake.
Using the same cookie or pastry cutter, cut out the middle of the final layer of the cake.
Secure your bottom layer to your cake board with a little buttercream from your bowl.
Fill the hole with your trick or treats.
Spread a little buttercream from your bowl around the top edge of the bottom layer and place the middle layer on top. again fill the hole with your trick or treats.
Spread a little more buttercream around the top edge of the middle layer.
Stack the trick or treats high in the middle so they sit above the middle layer.
Place the final layer cake on top making sure all of the trick or treats are hidden in the middle of the cake.
Using the remaining buttercream in your bowl, crumb coat the whole cake then finish with a smooth layer of buttercream.
Pipe large roses around the base of the cake.
Finish the cake by piping roses on the top of the cake, starting at the centre and working outwards.
Finish any gaps or edges with piped stars.