Preheat the oven to 170C / 340F (150C fan-assisted).

Grease and line a 20cm x 28cm (8 inch x 11 inch) non-stick shallow rectangular cake tin.

Turn the oven up to to 180C/350F (160C fan).

Put the butter and chopped dark chocolate in a large, 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.

Add the sugar and whisk 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 and gently fold through until evenly combined.

Pour the brownie batter into the tin and level off.

Bake for 35-40 minutes until set all the way across. A skewer pushed into the centre should reveal sticky crumbs.

Place the tin on a wire rack to cool, then place in the fridge to chill overnight.

Slice into 15 neat squares in a 5 x 3 pattern with a sharp, serrated knife.