Measure sifted flour, add baking powder and salt and sift again. Melt butter and chocolate over boiling water. Add sugar gradually to eggs, beating thoroughly. Then add chocolate mixture and blend. Add flour and mix well. Add nuts and vanilla and mix.
Bake in greased 8 x 8 x 2 inch pan at 350 degrees for 35 minutes. Cut into squares. Remove from pan and cool.