Because of the flaky, delicate nature of croissants, toasting them in a traditional toaster isn’t the best way to heat them. This is because, first, it is easy for the croissant to catch on the heating elements and tear, both ruining your croissant and creating a possible fire hazard. Second, it’s hard to tell how high to set your heat level to avoid burning. A lot of standard toasters don’t have slots big enough to fit a croissant either.
We suggest using either a traditional oven or a toaster oven for croissants. Both of these have windows that let you watch your croissant to ensure it doesn’t overcook. Both also cook croissants without squishing or tearing them. Using a toaster oven also lets you bypass a tray or baking sheet so you can get a good, golden color on both sides of the croissant without having to flip it over.