Combine white chocolate and butter in a heatproof bowl and place it over a saucepan with barely simmering water. Stir until completely melted and smooth. Or melt in the microwave at 50% power in short bursts with frequent stirring.
Combine eggs, sugar and salt in a large bowl and whisk or beat with an electric hand mixer until pale and thick. Mix in vanilla. Mix in warm melted white chocolate mixture. Add flour and fold it in gently. Just before all of the flour is incorporated, fold in the chocolate chips.
Spread the rest evenly into prepared pan. Bake for 25-30 minutes until the top is shiny and crackly.