Photograph: Jan Baldwin
Line an 18cm square tin with greaseproof paper.
Put the butter, caster sugar and condensed milk in a microwave-safe bowl - do not stir.
Microwave for 1 minute on full power (900W) and stir. Repeat this, heating and stirring 9 times, then stir briskly until cooled and thickened.
Stir in the vanilla extract and two-thirds of the pack of festive fruit and nuts baking addition.
Pour into the tin, then scatter over the remaining fruit and nuts. Cool, then cut into squares.