Triple Chocolate Brownies

4 oz. unsweetened chocolate, finely chopped
8 tbsp. unsalted butter, softened
1-1/2 cups sugar
4 eggs
1 tsp. vanilla
2 oz. white chocolate, chopped into ¼ inch pieces
2 oz. bittersweet chocolate, chopped into ¼ inch pieces
2 oz. milk chocolate, chopped into ¼ inch pieces
¼ tsp. salt
¼ cup plus 1 tbsp. flour, sifted
2 tbsp. cocoa powder

Melt the unsweetened chocolate in a double boiler over hot water. Whisk until smooth.

Cream together the butter and sugar. Stir in the eggs, 2 at a time, and mix just until incorporated. Stir in the melted unsweetened chocolate and add the vanilla. Stir in the chopped white, bittersweet and milk chocolate pieces. Add the salt, flour and cocoa powder; do not overmix.  

Spread the batter into a buttered and floured 9 inch square pan. Bake at 325º until almost completely set, 35 to 40 minutes. They should still be moist in the middle. Do not overbake. Let cool before cutting.