The 10th anniversary cake is an item that was released on January 4, 2011 in celebration of RuneScape's 10 year anniversary.
The cake has a "Celebrate" which when chosen, performs an emote with the cake.
Whenever a player is creating the cake, it requires 10 anniversary candles that are dropped from various monsters located in RuneScape and a 10th anniversary cake, which was also dropped by monsters (more often after the player has collected 10+ candles.) You cannot eat this item in the current state, although you can eat it before you apply the candles. It heals 10 life points. If a player wears a cape while doing the emote, the cape covers the candles and they become lit.