Can someone have a birthday cake and use a number candle or small birthday candles without lighting them on Shabbos? Meaning, can they prepare the candles and designate them as a cake “decoration” before Shabbos and therefore its not muktza?
Without getting into the issue of Kosaiv, because the candles are being attached to the cake on Shabbos, regarding muktza it is permitted to move such a candle. This is because this would give the candle a “Shabbos use”, to decorate the cake. This would be true even if the candle was not put into the cake before Shabbos, as it has a permitted Shabbos use. Understandably, the candle may not be lit on Shabbos. It is important to note however that this is without getting in to the issue of kosaiv.
The fact that the candle has a significant permitted use, to decorate the cake, would make the candle a kli shemilachto l’issur, and then it may be moved, “ltzorech gufo”, which would include moving it to decorate the cake. In fact, there is even room to argue that it might be a li shmilachto l’heter because perhaps we can say that the main use of the candle is as a decoration. Poskim.