Melt chocolate pieces and butter over hot (not boiling) water. Beat eggs until light and fluffy; gradually beat in sugar. Stir in oats, baking powder, salt, vanilla, walnuts and chocolate pieces. Pour batter into greased 20 cm square baking pan. Bake in preheated moderate oven (350 F.) 30 to 35 minutes. Cool; cut in squares. Sprinkle with confectioners' sugar, if desired.