Preheat oven to 170 degrees C (fan-forced). Grease and line an 18 x 28 cm slice tin.
In a bowl, mix cake mix, crumbled weetbix, melted butter and cocunut until well combined.
Press into the tin, smoothing the top with the back of a spoon.
Bake for 15-20 minutes, or until lightly golden. Cool completely.
For the icing, mix all ingredients together, then spread over the cooled slice. Sprinkle with a little extra coconut, set in the fridge, and cut into squares.