Combine 1/2 cup butter or margarine and chocolate in saucepan and melt over low heat. Beat white sugar and eggs together, add chocolate mixture. Stir. Add 1/2 cup drained pineapple and blend. Measure in flour, vanilla, baking powder, baking soda, salt, nuts and give it a good stirring.
Scrape into greased 9 x 9 inch pan. Bake in 350 degrees F (175 degrees C) oven for 30 minutes until edges show signs of pulling away from the pan. Cool. Frost with Pineapple Icing.
To Make Pineapple Icing: Combine confectioners' sugar, 3 tablespoons butter or margarine and 3 tablespoons pineapple in bowl and beat well, adding pineapple juice as needed for spreading. Smooth over brownies. Cut when set.