Place the butter, golden syrup, brown sugar (and salt, if using) into a microwave-safe bowl. Melt on 50% power for 4 minutes (stirring every minute).
Add the quick oats to the bowl and stir until all of the oats are coated in the golden syrup mixture.
Press the mixture firmly into the prepared baking tin. Bake for 30 minutes or until golden on top. Allow to cool completely.
Place the milk chocolate and coconut oil into a microwave-safe bowl. Melt for 3 minutes, 50% power, or until just melted. Pour the melted chocolate over the slice. Place into the fridge for a minimum of 1 hour before cutting into pieces.