Grease and line a 20cm x 28cm (8 inch x 11 inch) non-stick shallow rectangular cake tin.
Preheat the oven to 180C/350F (160C/320F fan).
Mix the espresso with 60ml/4tbsp of boiling water and set aside to cool.
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.
Add the beaten egg and stir with your whisk.
Stir in the coffee and vanilla
Sift the flour into the bowl and gently fold through until evenly combined.
Pour the brownie batter into the lined 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 on a wire rack to cool completely.
Place in the fridge to chill overnight.
Slice into 15 neat brownies in a 5 x 3 pattern with a sharp, serrated knife.