Step 2 – Using a melon baller (see tip), scoop balls from rockmelon and honeydew melon. Place on a large plate lined with paper towel. Cover with paper towel and pat dry.
Step 3 – Combine caster sugar and 1/4 cup hot water in a small heavy-based saucepan over low heat. Cook, stirring, for 4 minutes or until sugar is dissolved. Increase heat to high. Bring to the boil. Boil, without stirring, for 8 minutes or until mixture turns golden. Set aside for 30 seconds for bubbles to subside.
Step 4 – Arrange melon balls in a large serving bowl. Drizzle over toffee. Stand at room temperature for 5 minutes or until toffee is set and cooled. Serve immediately with yoghurt mixture.