Preheat oven to 180°C/160°C fan-forced. Grease a 16 x 26cm (base size) slice pan and line with baking paper, overhanging the 2 long sides.
Place flour, coconut, sugar and Weet-Bix in a large bowl and make a well in the centre. Add the melted butter to the well and stir to combine. Spread mixture over base of the prepared pan, pressing with the back of a spoon to evenly cover. Bake for 12 minutes or until lightly golden. Transfer the pan to a wire rack to cool completely.
Meanwhile, combine the mango flesh, condensed milk, lime juice and salt in a food processor. Process until smooth and combined. Pour over the Weet-Bix base. Bake for 25 minutes or until set and the edges are slightly bubbling. Set aside in the pan to cool to room temperature, then cover and place in the fridge to chill overnight.
Run a knife along the short ends of the slice to loosen. Use paper to lift the slice from the pan, then cut into small squares to serve.
