Preheat your oven to 180°C (160°C Fan, gas mark 4) and line a 9 by 13 inch baking tin with parchment paper.
Melt butter and chocolate together in the microwave or in a pan and set it aside to cool down.
230 g Butter, 200 g Dark Chocolate
Place caster sugar, light brown sugar, eggs and vanilla extract into a large mixing bowl. With an electric whisk, whisk the mixture until it becomes pale, frothy and has doubled in size.
200 g Caster Sugar, 140 g Light Brown Sugar, 5 Medium Eggs, 1 tsp Vanilla Extract
Pour an espresso shot (that's cooled down) and the dark chocolate and melted butter mixture into the whisked eggs mixture. Whisk together until combined.
30 ml One Espresso Shot
Sift plain flour, salt and cocoa powder over the mixture, then whisk again until it's evenly combined.
120 g Plain Flour, 50 g Cocoa Powder, ½ tsp Salt
Pour the brownie mixture into a lined baking tray and spread evenly using a spatula or spoon.
Bake it in the oven for 25 minutes and then leave it to cool down for at least 30 minutes. In this time, you can make the mascarpone coffee cream.