Measure the black cocoa powder into a medium heatproof bowl and pour over the boiling water. Stir thoroughly to combine - a thick paste should form. Cover and set aside to cool.
Whip the butter on medium-high using the paddle attachment or your hand beaters until it is lightened in colour and starting to get airy. Add the cooled black cocoa powder paste with the speed on low. Scrape down the bowl and continue to beat until combined - the butter should have turned a grey/black colour.
Add the powdered sugar in increments while the mixer is on a low speed. Continue scraping down, adding sugar and processing until all the sugar is added. The mixture should be a light black or dark grey colour and have a fluffy, buttercream texture.
Add the cream and salt and mix again on low to combine. If your buttercream is looking thick, you can add an extra tablespoon or more of cream until you reach your desired consistency.
Once you are happy with the buttercream consistency, remove it from the stand mixer. If it is looking or feeling soft, pop it in the fridge for 10 minutes to firm up.
Use your immersion blender/soup stick to blitz the buttercream. You should notice that it is immediately a darker black colour if you lift the blade. Continue until you have emulsified all the buttercream.
If the buttercream is still a little grey, take about 1/4 - 1/3 cup and microwave it for 3-4 seconds. Return the melty buttercream to the bowl and use a spatula to fold it into the mixture. This should further deepen the black colour.
Use to generously frost 12+ cupcakes or 1 large cake. I estimate you could very thinly frost two 20cm/8 inch cakes with this batch, although I'd recommend making either 1 1/2 - 2 batches to be on the safe side.