Beat egg whites until stiff.
Add castor sugar.
Add egg yolks, oil and vanilla essence.
Mix together cocoa and boiling water.
Separately combine flour and baking powder.
Alternately add the cocoa and flour mixture and fold in.
Spoon cake batter into cupcake pans.
Bake at 180 degrees Celsius for approximately 15 minutes.
Once cakes have cooled, pipe with frosting.
Beat butter and icing sugar until fluffy. Add nutella, cocoa and milk and beat in. If too wet add more icing sugar. If too stiff, add more milk. Spoon into piping bag and pipe onto cupcakes. (Top with chopped hazelnuts)