Amazing Peanut Butter Cookies With a Sourdough Discard Flax Egg

A dozen peanut butter cookies made with a sourdough discard flax egg cools in a cooling rack. The rack sits on a white and grey marble background.
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Egg shortage or not, you’ll want to bake these peanut butter cookies with a sourdough discard flax egg!

I have wanted to bake sourdough cookies for many years and every attempt has resulted in ho-hum cookies. Sourdough starter contains water and cookies do not and therein lies the problem.

Then I started baking cookies with flax eggs. For each egg I replace, I make a flax egg that contains 1 tablespoon of ground flaxseed and 2½ tablespoons of water. Wait, what, water? Maybe I could sneak a bit of sourdough into cookie dough via a discard flax egg. I tried it and this egg substitution works so well, I use it even if I have two dozen eggs on hand.

Speaking of which, I am fortunate to be able to buy pastured eggs at the farmers’ market, where eggs are plentiful during the shortage. Hens inhumanely packed together on factory farms to boost profits have created the ideal conditions for the bird flu to infect entire flocks in one fell swoop. Smaller farms have been much less affected and in some cases, their eggs now cost less.

A dozen peanut butter cookies made with a sourdough discard flax egg cools in a cooling rack. The rack sits on a white and grey marble background.
Peanut butter cookies baked with a sourdough discard flax egg

How to rework a recipe for a sourdough discard flax egg substitution

Start with a cookie recipe that calls for one or two eggs—and flour. Good contenders include peanut butter cookies (see above pic!), chocolate chip cookies, oatmeal cookies, sugar cookies, snickerdoodles and so on.

You wouldn’t want to make something like a flourless chocolate cake with these egg replacers or a soufflé or anything else that relies on eggs for structure. But for binding cookie dough, you may never go back to chicken eggs. You’ll save money and if you run out of eggs, you can still bake!

Recipe for one sourdough discard flax egg

  • 3 tablespoons sourdough discard, 100 percent hydration (47 grams)
  • 1 tablespoon flaxseed meal (8 grams)
  1. Combine ingredients in a small bowl. Let the mixture sit for 10 to 15 minutes to thicken. It will become quite stiff.
  2. Adjust the original cookie recipe by reducing the flour by 3 tablespoons for each sourdough discard flax egg added.
  3. Mix the sourdough discard flax egg into the ingredients at the recipe’s egg step.

Follow the cookie recipe steps as usual when substituting the egg

I combine the ingredients for these peanut butter cookies with a sourdough discard flax egg the same way I would with a standard egg. I cream the butter and peanut butter, add the egg substitute and then mix in the dry ingredients.

Ferment the peanut butter cookie dough if desired

After mixing the dough, roll it into balls and bake immediately or let it sit and ferment on the counter at room temperature. (Go here for a brief overview of fermentation.)

A large metal bowl of peanut butter cookie dough made with a sourdough discard flax egg sits on a white tiled counter to ferment. The bowl is covered with a metal pot lid. In the background are a red checked tea towel, a large mug, a teapot and two large jars of flour.
Peanut butter cookie dough fermenting for a short time on the kitchen counter

If you decide to ferment this peanut butter cookie dough with its smidgeon of sourdough, the good microbes in the discard will multiply. I ferment this for only a short while (four to six hours) so the microbes won’t have time to produce a huge colony in there. The dough won’t bubble up like a sourdough starter.

Fermentation doesn’t magically transform these into health food—they are cookies! But fermentation does help with digestion. And I digest a lot of these.

Two metal trays of peanut butter cookies made with a sourdough discard flax egg bake in the oven. A pizza stone is sitting on the bottom rack of the oven.
Peanut butter cookies baking in the oven
A dozen peanut butter cookies made with a sourdough discard flax egg cools in a cooling rack. The rack sits on a white and grey marble background.
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4.75 from 4 votes

Peanut Butter Cookies with Sourdough Discard Flax Egg Replacer

These classic peanut butter cookies call for a combination of flaxseed meal and a small amount of sourdough discard to replace an egg
Prep Time20 mins
Cook Time12 mins
Course: Dessert
Cuisine: American
Servings: 24 cookies


  • 1 hand mixer or stand mixer


  • 3 tablespoons unfed sourdough discard, 100 percent hydration *(see note) straight from the refrigerator
  • 1 tablespoon ground flaxseed brown or golden, finely ground
  • 1 cup all-purpose flour
  • ½ teaspoon baking powder
  • ½ teaspoon baking soda
  • ¼ teaspoon salt omit if using salted peanut butter
  • ½ cup butter, softened 1 stick, dairy or vegan
  • ½ cup unsalted peanut butter
  • ½ cup granulated sugar
  • ½ cup packed brown sugar golden or dark
  • ½ teaspoon vanilla


  • Make a sourdough discard flax egg by combining the sourdough discard and flaxseed meal in a small bowl. Set aside to thicken for 10 to 15 minutes. Preheat the oven to 375°F.
  • In a medium-size bowl, sift together the flour, baking powder, baking soda and salt.
  • In a large bowl, beat together the softened butter and peanut butter with a hand mixer on high speed until creamy, about a minute. Add the sugars and beat until creamy. Beat in the sourdough discard flax egg and vanilla.
  • Mix the dry ingredients into the wet ingredients. If necessary, use a clean hand to incorporate the remaining flour. If desired, cover the bowl with a plate and let sit on the counter to ferment for 4 to 6 hours.
  • Roll the dough into 1-inch balls and place 2 inches apart on ungreased cookie sheets. Flatten with the tines of a fork to make a criss-cross pattern. Bake for 10 to 12 minutes or until the bottoms have slightly browned, rotating the trays halfway through baking.
  • Allow to cool on the cookie sheets for 2 minutes. Transfer to a cooling rack to completely cool. Store in a container for a week or the freezer for six months.


This recipe calls for discard from a 100 percent hydration starter. That means the starter contains equal amounts of flour and water by weight. If your starter is, say, at 85 percent hydration, the dough may be slightly drier. If the dough is dry and does not come together, add a splash of liquid (water or milk of choice).  

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