Heat oven to 180°C (160°C fan forced). Meanwhile, grease a 20cm square tin with butter and line with 2 long strips of folded baking paper.
Place the butter and chocolate in a medium-sized heatproof bowl and set over a pan of simmering water, stirring occasionally until melted. Alternatively, microwave at 30 second intervals until melted. Leave the melted chocolate mixture to cool to room temperature.
Break the eggs into a large bowl and add the caster sugar. With an electric mixer on maximum speed, whisk the eggs and sugar for 5-8 mins or until thick and creamy and doubled in size.
Pour the cooled chocolate mixture over the egg mixture and gently fold together.
Sieve the flour & cocoa into the wet ingredients. Fold the mixture gently with a spatula until just combined (do not over-mix).
Pour the mixture into the prepared tin and carefully level the mixture with the spatula. Place in the middle of the oven and bake for 30 minutes.
In a small bowl, sift the icing sugar and cocoa together. Add boiling water and stir to a pouring consistency. Spread over the top of the cooled brownie.
To decorate: scatter sprinkles, m&m’s and candy eyes over the chocolate icing. Melt the white chocolate in the microwave in 30 second bursts and mix well with a drop of food colouring. With a teaspoon, drizzle over the top of the brownie.