Preheat oven to 180C and line and grease a lamington tin or shallow baking tray with baking paper.
Whisk together the flour, sugar, salt, bicarb soda and baking powder in a large bowl.
In a large jug whisk together the eggs, oil, buttermilk and vanilla, then gently fold the wet ingredients into the dry and mix until combined.
Transfer mixture to prepared baking tray and bake for 35 minutes or until just cooked through (press down the top of the slice and it should feel springy).
Let cool in tray.
For the icing - combine all ingredients in a small bowl and whisk until smooth and thick.
Drizzle over the top of the slice and sprinkle with the coconut. Let icing set before cutting into squares.