Preheat oven to 350 degrees and butter pan (15 x 10 x 1-inch). Melt 2 sticks butter with 4 squares chocolate and cool. Beat eggs with sugar. Add cooled chocolate mixture. Sift together flour, baking powder and salt. Add to egg mixture. Stir in vanilla and chopped nuts. Pour into pan. Bake at 350 degrees for 20 to 22 minutes. Do not let fudge squares get too dry. Icing: Melt chocolate with butter. Stir milk into sifted powdered sugar; add chocolate-butter mixture and stir until smooth. Spread on squares. Makes 40 to 48 squares.