Heat the oven to 180C/355F/Gas 4.
Lightly butter a 20cm x 30cm/8in x 12in straight-sided baking tin and line the base with greaseproof paper.
Finely slice the chillies on the diagonal.
Combine the sugar, water and chillies and bring to the boil, stirring. Simmer for five minutes.
In a heatproof bowl set over a pot of barely simmering water, melt the butter with 150g/5oz of the white chocolate, whisking well until smooth. Remove from the heat and allow to cool slightly.
In a separate bowl, beat the eggs, sugar and vanilla together until pale. Beat in the melted white chocolate mixture.
Fold in the sifted flour and salt, then the remaining chopped white chocolate. Drain the chillies and add most of them to the batter.
Pour into the baking pan, strew the remaining chillies on top, and bake for 25 minutes or until the top is firm and lightly golden. Remove and cool in the pan.
Cut into small or large squares, dust with icing sugar and serve.