Preheat oven to 165C. Brush a 26-centimetre round springform tin with extra melted butter and line base with baking paper.

Combine the chocolate and butter in a bowl over a saucepan of simmering water (do not allow bowl to touch the water). Stir until melted, then take off the heat and set aside to cool.

Stir egg yolks into chocolate mixture one at a time. Add coffee and Cointreau and stir. Add flour and mix until smooth.

Place egg whites in a large bowl with salt and beat until soft peaks form. Gradually pour in sugar and beat until thick and glossy. Gently fold egg whites into chocolate mixture in three batches.

Pour mixture into prepared tin and bake for 1 hour or until a skewer inserted in the centre comes out moist. Turn oven off and leave cake inside, with the door slightly ajar, to cool slowly for an hour. Allow cake to rest at room temperature for 30 minutes before carefully unmoulding.

To serve, dust with cocoa and serve with a dollop of mascarpone.