Preheat oven to 375 degrees F. Line with paper liners 12 muffins cups. Set aside.
Combine the boiling water and cocoa powder in a small bowl. Stir to dissolve and leave to cool to room temperate.
In a separate bowl, combine the flour, baking powder and salt.
In the bowl of your electric mixer, beat the butter on high for just a minute. Add all the sugar and beat on high until it's pale and fluffy, about 3 minutes. Add the eggs, one at a time, beating well after each addition. Beat in the vanilla extract.
Add the flour to the batter and mix until just combined, about a minute.
Pour in the chocolate mixture and mix until combined. Again, just for about a minute.
Evenly pour batter into prepared muffins tins and bake for about 15-18 minutes or when a toothpick inserted into the center comes out clean. Remove from oven and place on a wire rack to cool completely.
Make the icing by placing the chopped chocolate in a double boiler set over simmering water. Stir until the chocolate melts and remove from heat.
In a medium bowl, beat the butter on high until smooth. Sift in the powdered sugar and mix until smooth. Beat in vanilla extract. Pour in the cooled chocolate and beat on medium speed until combined. Turn speed to high and mix for a further 2 minutes or so until the frosting is a little glossy.
Put the icing in a piping bag and pipe it on the top of each cupcake as desired. You could also use a small knife or spatula to spread the icing over the top. Finish off the cupcakes with a Halloween decoration of your choice.
Will keep in an airtight container in the fridge for about 3 days.