Marshmallow-Topped Chocolate Fudge

1. Grease a 17.5cm square cake tin or baking dish with sunflower oil and line with non-stick baking paper, leaving a 5cm excess hanging over the edges to help you lift it out later.
2. Divide the condensed milk evenly between two medium bowls. Finely chop the white chocolate and add to one bowl along with the vanilla seeds, extract or paste. Melt the white chocolate in the microwave in 30-second blasts, stirring between each go, or set over a pan of simmering water, shallow enough that the water doesn’t touch the bowl. Once melted and smooth, remove and pour into the prepared tin, spreading it evenly.
3. Finely chop the milk or dark chocolate (or both) and add to the second bowl. Melt in the same way as the white chocolate, then spoon this in blobs all over the top of the white chocolate layer in the tin before spreading it evenly. Working quickly before the mixture sets, sprinkle the marshmallows in a single even layer on top, pressing them down gently to stick. Chill in the fridge for at least 3 or 4 hours, until set firm.
4. Lift the slab out of the tin and peel off the paper. Using a long sharp knife dipped in boiling water and wiped dry between each cut, slice into 36 pieces (six even-sized strips each way). These will keep for up to one week in the fridge in an airtight container, layered between parchment paper.

 Tip: Marble the white and dark layers together with a kitchen knife if you like a fun finish.