Makes 5 dozen.
6 ounces bittersweet or semisweet chocolate
1 cup unsalted butter
1 1/4 cups all-purpose flour
2/3 cup unsweetened cocoa powder
1 1/2 teaspoons baking powder
1/4 teaspoon salt
2 1/4 cups granulated sugar
2 teaspoons vanilla extract
1 cup (6 ounces) white chocolate chips
1 1/4 cups macadamia nuts, chopped
Preheat the oven to 350 degrees F. Line a 10 x 15 inch pan (jellyroll) with aluminum foil, shiny side up, and grease lightly.
In the top pan of a double boiler, combine the chocolate and butter. Place over the lower pan of hot water and heat until melted, then stir until smooth and let cool. In a bowl, stir together the flour, cocoa, baking powder, and salt. In the large bowl of an electric stand mixer fitted with a whip attachment, beat the eggs until light and fluffy. Beat in the sugar and vanilla, mixing well. Stir in the melted chocolate and the flour mixture. Stir in the white chocolate chips and 3/4 cup of the nuts. Spread in the prepared pan and sprinkle with the remaining nuts. Bake for 25 to 30 minutes, or just until barely set. Let cool in the pan on a rack, then cut into 1 1/2-inch squares.
Store the brownies in an airtight container for up to 4 days.