I made the cake for my friend's son yesterday. I finished off the icing today before taking it to the party. I had a flash of inspiration, and decided to make a number 5 in the sprinkles. I printed out a big number 5, then traced it onto a big piece of greaseproof paper. I carefully cut the 5 out to leave a stencil, then also cut a 5 shape out as a second stencil.
I placed the 5 shape in the middle of the cake, then sprinkled chocolate vermicelli all over. When the 5 was removed, I was left with plain icing only in that area. I then placed the 5 stencil over the cake, leaving just the 5 of icing showing. I carefully pressed star shaped sprinkles into the icing to make the 5, it turned out pretty well.