Preheat the oven to 170 degrees C. Grease a 20x30cm baking dish.
In a medium bowl, mix together the margarine and 1/2 cup of brown sugar until smooth. Stir in 1 cup of flour and salt until well blended. Press into the prepared pan.
Bake the crust for 15 minutes in the preheated oven, or until delicately browned. After removing the crust from the oven, increase the heat to 180 degrees C.
Make the filling while the crust is baking. In a medium bowl, stir together 1 cup brown sugar, baking powder and 2 tablespoons of flour until well blended. Mix in eggs and vanilla, beating until thick and foamy. Stir in the coconut and chopped nuts. Spread over the warm crust.
Bake for 25 minutes in the preheated oven, or until lightly browned and puffy. Cool completely, and cut into small bars.