Heat oven to 180C/160C fan/gas 4. Line 2 muffin tins with 16 paper muffin cases. Whisk together 175ml boiling water with the cocoa powder in a medium-sized bowl until smooth. Allow the mixture to cool until it reaches room temperature (put it in the freezer to speed it up), then whisk it with the eggs and vanilla.
In an electric mixer, combine the flour, salt, bicarbonate of soda and sugar. Mix on a low speed for 30 secs, then add one-third of the cocoa liquid and softened butter. Keep mixing on a low speed, then turn up to medium high and beat for 1 min. Add the remaining liquid in 3 batches, beating each one
for 30 secs. Scrape the sides and bottom of the bowl with a spatula and mix one more time. Fill the cupcake liners three-quarters full and bake for 20 mins, then remove and let them cool in the tin.
To make the frosting, beat the butter and cream cheese for 2-3 mins until creamy, then add the vanilla. While still mixing, pour in the sugar in 4 batches, beating each time to fully combine. Beat for a further min, then chill for 2 hrs.
Swirl the frosting over the cooled cupcakes and decorate with silver balls or whatever you like.