2) Put the butter, root beer and cocoa in a microwave proof bowl, cover with cling film and microwave on high for about 2 mins, until the butter melts. Stir to combine. Alternatively, melt together in a saucepan over a low heat.
3) Sift the flour, sugar, baking powder, bicarbonate of soda and salt together in a large bowl.
4) In a separate bowl, mix together the sour cream, eggs and vanilla.
5) Whisk the hot cocoa mixture into the dry ingredients. Stir in the sour cream mixture just to combine; take care not to overmix. Divide the batter evenly between the prepared muffin cases, filling each one about 3/4 full. Bake cupcakes until a toothpick inserted in the centre of the cake comes out clean, about 25 mins. Cool the cupcakes completely in the tin on a rack. Remove from liners and return cakes to a rack.
6) For the icing: Bring about 5cm of water to the boil in a pan large enough to fit a mixer's heat proof bowl above the water. Whisk the egg whites, sugar, cream of tartar, and salt in the bowl by hand. Set the bowl above the boiling water and continue whisking until the mixture is hot to the touch and the sugar dissolves, about 1 to 2 mins. Transfer the mixture to a standing mixer fitted with the whisk attachment and beat at medium-high speed until eggs beat up into a cool stiff meringue, about 5 mins. Add a few drops of desired essence and/or food colouring and lightly beat into the meringue.
7) For decorating: Using a spoon, offset spatula or piping bag, dollop, spread or pipe meringue on top of each cupcake to make a ghost or monster of your choice. Decorate with candied eyeballs or other candies. These can be finished 2 to 3 hrs in advance and held at room temperature.