How Do I Make Spikey Icing on top of Christmas Cake?

[BaconAndAvocado]

Usually I cover my cake with the roll-out icing but this year I'd like to try the old-fashioned icing where it sort of spikes up in places.

How is it done?

this is the Mary berry recipe I use

Love royal icing!

Royal Icing. Delia has a good recipe - Here. The longer you leave it, the harder it gets - if you want that tooth-cracking experience, ice well ahead of Christmas Eve (which is usually when I get round to it).

Tate and Lyle also do a royal icing powder - think it has egg white powder in it, so assume you just add water?