Stencilling numbers on a birthday cake, instead of piping, couldn't be easier, and it's a really nice way to avoid trying to write on a cake with melted chocolate.
Piping is hard. Stencilling is easy.
First things first: you need a stencil. I opted to use a word processor, choosing a pretty font, typing up the numbers I needed on my computer, and printing them on regular white paper. Remember to make them big enough, but not too big to fit the top of your cake.
Overall, I think this is a pretty easy way to decorate a cake and a fun way to celebrate a birthday.
If you are looking for a smoother look to the numbers, let the cake chill for about 5 minutes in the fridge/freezer before beginning the stencilling process. Chilling hardens the surface of the frosting a little so that the stencil will peel off more cleanly without pulling up some of the frosting. I did this tutorial without any refrigeration, and it still worked out pretty well, all things considered.
Now, go forth and stencil some cakes!