Preheat oven to 180°c/160°c fan-forced. Line 16 holes from two muffin pans with paper cases. Place cocoa and brown sugar into a medium bowl, add boiling water and stir until combined, set aside.
Beat 125g butter and caster sugar until fluffy. Add vanilla, eggs, flour, baking powder and bicarbonate soda. Fold in cocoa mixture. Divide into cases; bake for 20-25 minutes or until cooked through; cool.
Use a small sharp knife to cut a 2cm across by 2cm deep cone from each cupcake. Trim off pointy end of each cone, leaving a flat circle of cake.
Before serving, squeeze sweetened condensed milk into a bowl, setting aside 2 Tbsp for later. Add 1 tsp of green food colouring to the bowl and stir to combine. Spoon 1 tsp into each cupcake cavity and replace circle of cake.
Beat 125g butter until creamy; add reserved sweetened condensed milk. Gradually add in icing sugar, until combined, beat on high until frosting is light and fluffy. Tint with food colouring. Place nozzle onto piping bag and transfer frosting. Pipe dots of frosting onto each cupcake and add eyeballs.