Preheat the oven to 350 degrees.
Butter and flour a 12 x 18 x 1-inch baking sheet.
Melt together the butter, 1 pound of chocolate chips, and the unsweetened chocolate in a medium bowl over simmering water. Allow to cool slightly. In a large bowl, stir (do not beat) together the eggs, coffee granules, vanilla, and sugar. Stir the warm chocolate mixture into the egg mixture and allow to cool to room temperature.
In a medium bowl, sift together 1 cup of flour, the baking powder, and salt. Add to the cooled chocolate mixture. Toss the walnuts and 12 ounces of chocolate chips in a medium bowl with 1/4 cup of flour, then add them to the chocolate batter. Pour into the baking sheet.
Bake for 20 minutes, then rap the baking sheet against the oven shelf to force the air to escape from between the pan and the brownie dough. Bake for about 15 minutes, until a toothpick comes out clean. Do not overbake! Allow to cool thoroughly, refrigerate, and cut into 20 large squares.