Preheat oven to 180 degrees C. Coat a 25 x 30 cm baking tray with non-stick cooking spray.
Melt chocolate and butter in bowl over simmering water. Remove from heat and stir until smooth.
Sift together the flour, baking powder and salt; set aside. In a medium bowl stir together the sugar, vanilla and eggs. Add the chocolate mixture and stir until well blended. Then add the dry ingredients; mix well. Finally stir in the nuts and 3/4 cup of the white chocolate chips.
Spread the batter evenly into the prepared tray. Sprinkle remaining white chocolate chips evenly over the top. Bake for 20-25 minutes until firm and white chips are lightly coloured. Cut into bars when cool.