Pre-heat your oven to 350F and line a small square pan with baking paper.
Combine the eggs and sugar in the bowl of your mixer and mix for 5 minutes until thickened, pale and fluffy.
In the meantime, melt the chocolate and butter in a bowl over a hot water bath.
Stir the melted chocolate into the egg mixture then fold in the flour, cocoa powder and salt.
Pour the batter in the prepared pan and bake in the oven for 10 minutes.