Grease and line a small tin (about 10x15cm / 4x6 inches). Since the tin is so small, the easiest way to bake it this is to cut a piece of baking paper a couple of inches bigger than your tray, then scrunch it up and unscrunch. It will then push into the corners of your tray easily.
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 30 second bursts and stirring after each burst. Alternatively, you can put an inch of water in a pan, place the bowl on top (so that the water isn't touching the bowl) and bring to the boil so that the steam gently melts the chocolate, stirring as you go.
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 and vanilla. Whisk together until smooth.
Sift the flour into the bowl. Gently fold through until evenly combined.
Stir in the chocolate chips, reserving a small handful to go on top (about 25g).
Fill with the brownie batter and level off.
Bake for 10-15 minutes until set all the way across. A skewer pushed into the centre should reveal sticky crumbs.
Place on a wire rack and scatter the remaining chocolate chips on top.
Once cooled completely, transfer to the refrigerator to chill overnight, then slice into four to serve.