I have a 'Mrs Beeton's' version for marzipan and I also saw Rachel Allen make her own on her 'Christmas Bake'. Rachel then cooked the cake with the marzipan on and it looked lovely. It may well be on this site somewhere.
Mrs Beeton's recipe -
1lb loaf sugar (granulated)
12oz ground almonds
3oz sifted icing sugar
2 whites of eggs
1.5 gill water
flavouring
Boil the sugar and water to 240F. Then move the pan from the heat, and when the syrup has cooled slighty add the almonds and egg whites. Stir over a low heat for a few minutes then turn onto a slab, stir in the icing sugar and work with a spatula until the mixture is cool enough to handle. Knead until smooth, add flavouring and mould to desired shape.
I think the flavouring is almond, but it does not state which flavouring to use.
Hope it helps