For the meringue
Preheat the oven to 180°C, gas mark 4. Grease a 20-cm square cake tin and line the base with baking paper.
To make the brownies, melt the butter and bring to a simmer. Pour the butter over the chopped chocolate in a large bowl and stir until smooth. Mix in the sugar, then leave to cool for a few minutes. Beat in the eggs, then sift in the flour, cocoa and coffee powder and fold in until the mixture is smooth and glossy.
Pour the mixture into the prepared cake tin and level the top.
For the meringue, in a large, clean and dry bowl, whisk the 4 egg whites to stiff peaks, adding the caster sugar a little at a time. Add the vanilla extract. Sift the cornflour, coffee and cocoa powder together, then fold into the meringue until even and glossy.
Spread the meringue over the brownie batter.
Bake in the oven for 25 minutes. Turn off the oven and leave the brownies inside for a further 5 minutes before removing. Serve warm.