Leave the ice cream on the bench to soften
Line a medium mixing bowl with plastic wrap
In a second larger mixing bowl combine the softened ice cream with the marshmallows
Scoop the Ice cream and marshmallow mixture into the lined mixing bowl, freeze. This stage can be done well in advance of the party
A day or two before the party, remove the ice cream and unwrap it. Place it on a large serving plate.
Take the Elsa doll, undress it, and remove the legs (these can be clicked back together after the party)
Take a hot spoon and scoop out a small well in the centre top of the ice cream, press the torso of Elsa into the hole. Place the cake back into the freezer
Prepare the buttercream icing
Start with softened butter (not melted, but soft). Use an electric beater or cake mixer to beat the butter until it is as white as possible. Then add ⅓ of the icing sugar and beat, add ⅓ of the milk, alternate adding the icing sugar and milk until it has all been added, flavour the icing with the vanilla
Separate the buttercream into 3 bowls, add blue food colouring to 2 of the bowls so that you end up with three colours of buttercream, 1 white, 1 light blue and 1 darker blue
Fill 3 piping bags with the buttercream icing, use a star tip
Take the Ice Cream out of the freezer, Pipe stars of icing on to the ice cream to create Elsa’s Dress, I did mine very haphazardly and I think it looks fine. If the ice cream begins to melt, place the cake back into the freezer for half an hour or so to re-harden and then continue piping.
Decorate the torso of the Elsa Doll to create an icing bodice for her dress
Freeze the Ice Cream Cake until you are ready to serve it