Writing on a cake is a fairly straightforward task, but there are a handful of tricks for doing it well and consistently.
First, practice! Buttercream and melted chocolate are both good options for writing, but they perform differently, so you’ll want to get used to whichever you choose. Use a pastry bag with a fine tip or a plastic bag with a corner snipped off and practice on a piece of parchment paper until you feel comfortable and have settled on a writing style. Steady your writing hand with the other hand as you move across the paper.
When you’re ready to write on a cake, use a toothpick to mark where your words should begin and end. You could even trace the letters onto the cake with the toothpick to use as a guide when you write.
It’s best to work on a chilled cake, which will make it easier to fix mistakes. If you do make a mistake, stick the cake back in the fridge until the writing is firm. Then, use the tip of a knife or a toothpick to carefully lift the writing off of the cake.