Preheat oven to 175°. and butter and flour a 9-inch square baking pan, knocking out excess flour.
In a double boiler or metal bowl set over a saucepan of barely simmering water melt white chocolate and butter, stirring, until smooth and remove top of double boiler or bowl from heat. Cool chocolate mixture to room temperature.
In a large bowl with an electric mixer beat eggs, sugar, and vanilla until thickened and pale and beat in white chocolate mixture. Into a bowl sift together flour, baking powder, and salt and beat into chocolate mixture just until batter is combined well. Stir in nuts and chocolate chunks and spread batter evenly in pan.
Bake brownies in middle of oven 30 to 35 minutes, or until a tester comes out clean. Cool brownies completely in pan on a rack before cutting into 16 squares. Brownies keep, layered between sheets of wax paper in an airtight container at cool room temperature, 5 days.