The one described in Larousse Gastronomique is similar to a Biscotti or Almond Macaroon, just a lot more sugar. But nowadays we use almond paste and that also has a lot of sugar. The crumbles of Krokant (croquant) you achieve by crushing it with a rolling pin. There are many products that can be used as possible substitutes, like Nougat and Torrance, just depends on how far you cook the sugar and the treatment of the product afterwards. You can even buy Krokant sprinkles to decorate the outside of tortes.