Pop the apples in a large bowl, pour over boiling water to cover and then quickly pour it off. This takes off any wax coating from the apples and will help the toffee to stick.
Polish dry, twist off the stalks and push wooden ice lolly sticks halfway into the core of the apples from the stalk end.
Combine the caster sugar and water in a large heavy-based pan and stir constantly over a low heat with a wooden spoon for about 5 minutes until the sugar has dissolved.
Add the golden syrup and butter and stir until melted and combined. Bring to the boil without stirring, otherwise the sugar may crystallise, until the mixture reaches 149°-154°C for about 15-18 minutes. (If you don’t have a thermometer, check the toffee is ready by dropping a teaspoon of the mixture into a bowl of cold water. It should harden instantly, and when removed, be brittle and easy to break. If you can still squash it, continue to boil.)
Remove the pan from the heat, let the bubbles subside a little, then tilt the pan, and quickly and carefully dip the apples one at a time in the toffee. Twirl as you dip, to completely cover. Let any excess drip away before placing on a paper-lined baking tray to harden. If you find the toffee becomes too thick because the temperature has dropped, simply heat again.
Once set, store the toffee apples in an airtight container in a dry place, not the fridge, and eat within 24 hours or the toffee will soften and start to liquefy.