Base: Melt the fat and Siúcra/McKinney's Icing Sugar in a saucepan. Mix in the crushed cakes. Press the mixture into a springform tin. Refrigerate.
Caramel filling: Mix together the milk, margarine and Siúcra/McKinney's Granulated Sugar and boil until thick, about 6 minutes. Allow the cream to cool slightly, then pour into the base. Refrigerate.
Chocolate filling: Combine the dark chocolate, whipping cream, Siúcra/McKinney's Granulated Sugar and butter in a saucepan and melt over a low heat. Allow to cool slightly then pour the mixture into the base. Refrigerate until ready to serve.
Decoration: Before serving, decorate the caramel and chocolate tart with the whipped cream and chocolate chips.
* Condensed milk: Boil together 1 litre milk and 150g Siúcra/McKinney Brown Sugar in a low, broad-bottomed saucepan (preferably Teflon) until about 400 ml remains. Takes about 30 minutes. The milk becomes light brown, thick and possibly crumbly.