Preheat oven to 350 degrees. Grease and flour a 13x18x1-1/2-inch sheet pan.
Melt together the butter, 1 pound chocolate chips and bitter chocolate on top of a double boiler. Cool slightly. Stir together the eggs, instant coffee, vanilla and sugar. Stir in the warm chocolate mixture and cool to room temperature.
Stir together 1 cup of the flour, baking powder and salt. Add to cooled chocolate mixture. Toss the walnuts and chocolate chips with 1/4 cup flour to coat. Then add to the chocolate batter. Pour into prepared pan.
Bake for about 30 minutes, or until tester just comes out clean. Halfway through the baking, rap the pan against the oven shelf to allow air to escape from between the pan and the brownie dough. Do not overbake! Cool thoroughly, refrigerate well and cut into squares.