Preheat oven to 190 degrees C. Grease 20x30cm baking dish. Place a 40cm length of greaseproof paper lengthwise in pan. Smooth paper so it conforms to pan, allowing excess to hang over ends of pan. Butter the paper.
In a medium bowl combine flour, baking powder and salt, set aside.
In a large mixing bowl beat butter, sugar and vanilla until light and fluffy. Add eggs one at a time, beating well after each addition.
Stir in flour mixture 1/2 cup at a time, blending well after each addition. Stir in chopped nuts. Spread evenly in prepared pan.
Bake 10 - 15 minutes or just until a toothpick inserted in the centre comes out clean. Holding ends of waxed paper, lift biscuit base out of pan and place on rack. Cool.
To prepare Glaze: In a large bowl whisk together icing sugar and cocoa until no lumps remain. Add vanilla and 4 tablespoons milk. Add enough of the remaining milk to make a smooth, creamy glaze of medium consistency.
Line a large baking tray with waxed paper. Place coconut on a large plate. Cut biscuit base into 48 strips.
Use a fork to spear lamingtons; dip into chocolate glaze, allowing excess to drip back into bowl. Place chocolate dipped lamington on plate of coconut; use 2 forks to turn so that all sides are coated. Place on prepared baking tray and refrigerate until glaze is set. Store in airtight container at room temperature one week; freeze for longer storage.