Preheat oven to 180ºC . Butter an 8 or 23 cm square baking pan. Line pan with with parchment paper or waxed paper. Combine melted chocolate with melted butter. With a whisk, beat eggs with sugar and vanilla. Beat in chocolate mixture. Stir flour with baking powder and salt. Stir flour mixture into chocolate mixture. Stir in white chocolate and pecans. Pour batter into prepared pan and bake 30 minutes. (Brownies may appear to be underbaked but will set when chilled). Cool. Do not worry if sides are slightly higher than middle. Just press sides down as brownies cool. Drizzle melted white and dark chocolate in a zigzag pattern on top uncut brownies. Allow to rest at room temperature half hour. Cut into squares. Chill.