In a medium bowl, cream together the butter and 1/2 cup brown sugar until smooth. Stir in the 1 1/2 cups of flour. Press the mixture firmly into the bottom of a 20x30cm pan. Bake the crust for 15 minutes in the preheated oven until golden.
In a medium bowl, mix together the eggs and 1 1/2 cups brown sugar. Stir in the vanilla. Combine the 1/4 cup flour, cocoa (if desired), baking powder and salt; stir into the egg mixture. Fold in the coconut and walnuts. Pour into the pre-baked crust.
Bake for 25 to 30 minutes in the preheated oven, until firm. Cool and cut into squares.