Preheat oven to 180℃.
Grease and line a 20 cm square baking pan with baking/parchment paper and set aside.
To a large bowl, add crushed Weet-Bix, flour, caster sugar, desiccated coconut and lemon zest and mix to combine.
3 Weet-Bix, 120 g plain flour (all-purpose flour), 165 g caster sugar, 75 g desiccated coconut, ¾ teaspoon baking powder, 1 large lemon - zest and juice
Add melted butter and mix until well combined.
Tip the base mixture into the prepared pan, and press down well to compact.
Bake for 15 minutes or until lightly golden, then allow to cool slightly while you make the topping.
For the topping: In a medium bowl, mix together sweetened condensed milk and lemon juice.
Spoon this mixture over the base and smooth it out using a silicone or offset spatula (see my notes).
Bake for 16 to 18 minutes or until the topping is starting to bubble at the edges and the centre is set.
Allow to cool in the tin for 10 minutes, then remove from the tin and place on a wire rack to cool completely, then slice.