Preheat the oven to 180°C.
Butter a 20cm x 30cm baking tin and line the base with greaseproof paper.
Slice the chillies diagonally.
Boil the sugar, water, and chillies, then simmer for five minutes.
Melt the butter and 150g of white chocolate in a bowl over simmering water until smooth.
Beat the eggs, sugar, and vanilla until pale, then stir in the melted chocolate.
Fold in the flour, salt, remaining chopped chocolate, and most of the drained chillies.
Pour the batter into the pan and top with the remaining chillies.
Bake for 25 minutes or until golden and firm, then cool in the pan.
Cut into squares and dust with icing sugar to serve.