In large bowl, whip egg whites with electric mixer at high speed until stiff peaks form. Set aside. In another large bowl, whip egg yolks, sugar, and extract at high speed until the color is a pale yellow. In a small bowl, melt butter and white chocolate at 50% power for 1 minute. Continuing microwaving in 30 second intervals at 50% power, stirring between each interval, until melted. Add white chocolate mixture to egg yolk mixture until combined. Add flour and salt until combined. Slowly pour in milk while mixing until mixture is smooth. Mixture will appear thin. Fold in 1/3 of the egg whites. Continue to add egg whites until mixture is fully combined and there are no large lumps. Pour mixture into pan.