Preheat oven to 170 C / Gas 3. Line a 33x23cm baking tin.
Melt chocolate and butter in a heatproof bowl set over a pan of barely simmering water.
Remove from heat, add sugar and mix well. Add flour, mix well, then add eggs and mix well until mixture is smooth and shiny. Spoon mixture into prepared tin.
Bake for 30 to 35 minutes. Leave to cool then slice and dust with icing sugar to decorate.