If you don't have ready-made toffee, boil the condensed milk and let it cool completely. Take the butter out of the fridge ahead of time so it softens.

In a bowl, whip the butter until fluffy, then add the toffee gradually and whip until you get a smooth, fluffy cream.

Spread the cream between all the wafer sheets and press them firmly together.

Spread cream on top of the cake too and decorate with wafer crumbs and walnuts.