Break up the Weetbix biscuits into small pieces, either with your hands or by placing them in a ziplock bag and gently crushing them.

In a saucepan or the microwave, melt the butter until it's completely liquid.

Pour the sweetened condensed milk into the melted butter and stir until the mixture is well combined.

In a large mixing bowl, combine the crushed Weetbix with the butter-condensed milk mixture. If you're adding coconut, stir it in now.

Grease a baking tin or slice tray and press the Weetbix mixture evenly into the tin, using the back of a spoon or your hands to flatten it.

Place the tin in the refrigerator and allow the slice to chill for at least 2 hours, or until it's set.

If you'd like to top your Weetbix slice with a layer of chocolate, melt the chocolate in a double boiler or the microwave, and pour it over the chilled slice, spreading it evenly.

Return the tin to the fridge and let the chocolate layer set, which should take another 30 minutes to an hour.

Once the slice is fully set, remove it from the tin and slice it into squares or rectangles. Serve chilled and enjoy!