Baking is, without a doubt, a science, and it's easy to get it wrong without proper measurements. One way of ensuring a perfect bake each time is by adding hard-boiled eggs.
Don’t put a whole hard-boiled egg in your baked goods. The trick is all about the yolk and incorporating it so that your dough is lighter and your finished treats are more tender.
A hard-boiled egg yolk is a welcome addition to your doughier baked goods because it impedes gluten — which gives your baked goods their chew — from forming.
To add hard-boiled egg yolks, grate the cooled yolk into the dry ingredients, allowing the yolk to mingle with the flour before you add wet ingredients so that gluten cannot form.
You’re not substituting your hard-boiled egg yolk for one egg already in the recipe — these yolk additions are on top of what the recipe already calls for.