"17 Egg Substitutes Every Baker Needs to Know" (allrecipes)

[La Coliniere]

4. Flax eggs work well in many baked goods.

Use 1 tablespoon of ground flax seeds (flax meal) to 3 tablespoons of warm water and let sit for 15 minutes until it thickens up. I usually put it in the freezer to set up. You can’t use this to replace scrambled eggs or omelettes of course, only in baked goods. It works even with boxed cake mixes and sweet breads. Works well with Aldi’s apple muffin/loaf and banana bread mixes, but also when baking from scratch. My wife recently made a batch of cut out cookies from scratch using flax eggs and they came out perfect.