Heat the oven to 180°C (160°C fan). Line a 20 × 30 cm slice tin with baking paper, leaving an overhang on the long sides to lift the slice out later.
Crush the Weet-Bix in a bowl with the back of a spoon until broken into rough crumbs, not powder. You want some pieces the size of rolled oats.
Add the flour, coconut, cocoa, baking powder and sugar to the same bowl and stir to combine.
Pour in the melted butter and mix until every bit of the dry mixture is damp. It will look like coarse crumbs rather than a batter — that’s correct.
Tip into the tin and press down firmly and evenly with the back of a spoon until the surface is compacted. Take your time here; this is what holds the slice together.
Bake for 20 minutes, until the surface is dry to the touch and the edges have just begun to pull away from the paper. It will still feel soft in the middle — it firms as it cools.
Make the icing while it bakes. Sift the icing sugar and cocoa into a bowl, add the softened butter, then add the boiling water a tablespoon at a time, beating until smooth and just pourable.
Leave the slice in the tin for 10 minutes, then spread the icing over the warm base and scatter with coconut.
Cool completely in the tin before lifting out and cutting into 24 pieces. Cutting warm will drag the icing.