Although you can simply use a round piping tip (about 1,2 cm - 1/2-inch) to pipe the eclair, using such tip will usually result in éclairs that crack in irregular ways. To allow for a nice round shape and perfect chocolate eclairs, the secret is to use an open star piping tip or french start tip. The grooves on the piped eclairs will direct the way they expend in the oven and give them an even shape.