Preheat your oven to 160°C. Grease and line a 23cm square baking tin with parchment paper.
For the brownies, melt together the chocolate and butter over a bain-marie or in short 30-second microwave stints. Stir to ensure it melts evenly.
Beat the sugars and eggs together briefly using a whisk. Then slowly pour in the melted chocolate mixture.
Stir through the flour and then tip into your tin. Bake in the oven for 30 minutes until risen but still a little soft to touch. Leave to cool completely in the tin.
When the brownies are cooling, you can make the topping. Add the ingredients for the icing to a heatproof bowl and set over a saucepan of simmering water.
Whisk continuously until the sugar has melted, the mix has reached 75°C and the granules have dissolved. Keep whisking until it comes up to temperature.
Transfer to a freestanding mixer and whisk until stiff peaks form.
Once the brownies have cooled, spread over the espresso marshmallow icing.