Butter and line a 30cm x 21cm tin. Preheat the oven to fan 160/conventional 180/gas 4. Break up the chocolate with the butter and melt in the microwave on Medium for about 5 minutes, stirring halfway through.
Beat the sugar and eggs in a bowl. Stir in the melted chocolate, add the flour and beat well. Pour into the tin and bake for 40-45 minutes, or until the top looks papery and feels slightly wobbly. Leave to cool in the tin.
Break up the chocolate with the butter and melt in the microwave on Medium for about 1 minute. Stir until smooth then spread over the cake. Dust with icing sugar and cut into squares.