Preheat oven to 325°F. Grease a 9 inch square baking pan. Line the bottom of the pan with parchment paper and set aside. Chop half of the white chocolate into very small pieces. Chop the remaining white chocolate and the bittersweet chocolate into large chunks, set aside. Sift together the flour, baking powder and salt. Melt the butter in a small saucepan. Do not allow the butter to bubble or to color. Remove from heat and add the finely chopped white chocolate. DO NOT STIR the chocolate in, set aside. Beat the sugar, eggs and vanilla together until thick and lemon colored, about 5 minutes. Lower the mixer speed and add the butter mixture. Mix just to blend. Add the flour mixture and mix just until flour is mixed in. Use a rubber spatula to fold in the two chocolates and the nuts. Scrape the batter into the prepared pan. Bake until the brownies are just set, about 30 minutes. A toothpick inserted in center, should come out with moist crumbs attached. As with all brownies it is better to under bake then over bake. Allow to cool about 1 hour and then cut into squares