You Should Never Substitute Butter For Egg Wash. Here's Why
It seems from time to time everyone happens to be short on an ingredient or two while cooking or baking. This especially seems to happen when you have accounted for everything you need in the dish itself, but maybe not the extras or toppings that you need to polish it off. One example could be having enough eggs to make some kind of pastry, but not an extra egg to make an egg wash to complete them. Usually, there are plenty of tricks and substitutes you can pull out to finish what you're making. However, butter is one substitute for egg wash you should really avoid.
According to Asking Lot, you definitely don't want to use butter for egg wash because the egg in the wash serves a specific purpose. It is actually the egg that acts as a binder for a lot of pastries that call for egg wash to seal the pastries. The egg helps make the flour stick, but it really helps flour bind to meat, like a beef wellington, for example. Butter, on the other hand, will melt when it encounters hot oil and can cause the breading to slide right off of the meat.
It depends on how you are using egg wash
But there's another reason people use egg wash that makes butter as a substitute even more complicated. Egg wash is often used to make toppings on pastries stick. It's also used as a glaze to help the pastry achieve the perfect golden brown color and shine. Butter will not help toppings stick to pastry since it isn't a binder like an egg is, but butter can work in place of an egg wash for some foods.
When butter is added to a pastry, it seeps in as it melts and can make it taste even better and richer (via Taste Essence). While it might taste great, the only time you can get away with using butter as a substitute for an egg wash is as a glaze. Butter will make the pastry softer and add a slight shine, but there are still drawbacks (via Our Everyday Life). Butter doesn't brown as well as an egg wash, so the overall pastry will be lighter in color. Keep in mind, this isn't a suitable substitute for glazing any pastries that should be crisp, either, given that the butter will soften it.
So, avoid using butter in place of an egg wash unless you plan to use it on pastries that can stand a little softening and a little less color.