Preheat the oven to 350°F/175°C. Place parchment paper on two baking sheets.
Place the flax seeds in a bowl with the 1/4 cup of water to soak. It will turn into a moist paste in about 5 minutes.
Combine butter and both sugars in the large mixing bowl of the stand mixer, until creamy. Add the egg yolks one by one, and add the vanilla extract and flax seeds.
In a separate bowl, mix the flour, brewer's yeast, wheat germ, baking soda, salt and cream of tartar and make sure they are well mixed.
Turn the mixer to a low setting. Add the dry-ingredient mixture to the wet mixture in thirds, to prevent the mixture from blowing out of the bowl too much. Mix until incorporated.
Scoop small balls of dough of the bowl with a spoon, ice cream scoop or your hands. Place on the baking sheet with about 2 inches between them as they will spread a little bit.
Bake for 10-14 minutes, until the edges are golden. Don't over bake, as this will make the cookies bitter.