Put the butter and chopped dark chocolate in a small, heatproof bowl. Gently melt until smooth. You can use the microwave to do this, using 5 second bursts and stirring after each burst. Be careful as it’s such a small amount it can burn very quickly.
Whisk the sugar in. The mixture will thicken - graininess at this stage is fine.
If the mixture is hot, wait until it's just slightly warm before adding the beaten egg white, vanilla and flour. Whisk together until smooth.
Stir in all but a tablespoon of chocolate chips.
Grease a small, microwave safe ramekin. It should be able to hold all of the mixture with 1½cm (½ inch) of clear space at the top. Spoon the batter in and level off.
Microwave on full power for 45 seconds , then push a skewer into the centre - it should reveal sticky crumbs. If it doesn’t, cook in further 10 second bursts until you have the right consistency. Remember everyone's microwave is different!
Place on a wire rack, top with the remaining chocolate chips and leave to cool for 5 minutes so that you don’t burn your mouth, then dig in.
Alternatively, you can allow the brownie to cool completely, then transfer to the refrigerator to chill overnight. You can then tip it out and pop in a lunchbox, if you like. If it’s stuck in the container, a knife around the inside of the dish, then run a little warm water over the bottom of the dish and the brownie should slide out.