Lay an 18cm square cake tin on a piece of greaseproof paper. Draw around it. Then cut out the square, just inside the line. Using a paper towel, wipe cooking oil all over the inside of the tin. Press in the paper square and wipe oil all over it too.
Sift the icing sugar through a sieve into a large bowl. Make a hollow in the middle of the sugar with a spoon.
Using clean scissors, cut the marshmallows in half. Put them into a small saucepan with the butter, milk and vanilla.
Gently heat the pan on a low heat, until everything has melted. stir the mixture every now and then.
Pour the mixture into the hollow in the icing sugar. Stir everything together really well until the fudge is smooth.
Put the fudge into the tin and push into the corners. Leave it in the tin to cool. Then chill it in a fridge for 3 hours.
Loosen the edges of the fudge with a blunt knife. Then, turn it out onto a chopping board and peel of the paper.
Carefully cut the fudge into 36 pieces, using a sharp knife. Then, put it back in the fridge for an hour to harden.
Store in an airtight container in a fridge and eat within a week ;)
Also you could decorate the fudge by putting a 'smartie' (or other sweet)on top. <3