For the cupcakes,
preheat the oven to 180°C. Place Halloween coloured paper liners into an 8-cake capacity muffin tin. Place the macadamias, flour, sugar, beetroot powder and baking powder into the bowl of a food processor and pulse until the mixture becomes a flour-like consistency. Do not over-process or it will start to stick together. Coarse macadamia pieces are ideal. Place the flour mixture into a large bowl.
In a separate bowl, lightly whisk together the eggs, butter, oil and milk. Add to the dry ingredients and stir to combine, taking care not to over-mix. Spoon into the cupcake liners so that they are ¾ full. Bake for 12-15 minutes, until puffed and cooked through when tested with a skewer. Cool in the tin for 10 minutes before removing to a wire rack to cool completely.
For the icing, combine all the ingredients in a bowl and beat on a medium speed until the mixture is combined and stiff peaks form, about 3 minutes. Do not overbeat or the cream will curdle.
To decorate, use the food decorating pens to turn each whole macadamia into a bloodshot eyeball – black for the pupil, blue for the iris and red squiggles for the bloodshot veins. Pipe icing onto each cake and drizzle over some red write-on gel. Top with a macadamia eyeball and serve.