Place the eggs and sugar into the bowl of an electric mixer and whisk for about 8 minutes - until tripled in volume and stable.
Whilst the eggs and sugar are beating, melt the chocolate and butter (separately) - take them off the heat when not quite fully melted so that they can finish melting and cool a little before getting mixed in.
Weigh the cocoa, flour and salt into a bowl, mix and set aside.
Once the egg mix is ready, add the butter and dry ingredients alternately (2-3 additions in total), whisking to combine between additions. Start on butter and finish on dry, ideally.
Fold in the melted chocolate. If you timed it right and it's not a particularly cold day, the mixture in the bowl should be more or less the same temperature as the chocolate - that's something that will help promote that lovely brownie skin.
Fill piping bag with mixture - if it's really runny (e.g. a particularly warm day), you can fridge it for 10 minutes to firm up a little. But don't let it solidify!
Pipe into well-greased mini cake moulds, leaving about 1cm of space at the top.
Bake for 18-20 minutes (until risen and starting to crack on top), then leave to cool and set in the tins for 10 minutes, then remove (they'll still be hot!!) to a wire rack to cool completely.
Delicious warm, room temp or cold. Alone, or with ice cream/cream. They keep really well - in an airtight box for 5-7 days, and they freeze a dream!