Preheat oven to 350.
Combine ground flax seeds and water, set aside until all water is absorbed. Meanwhile, continue with next steps.
In a stand mixer with the paddle attachment, combine softened butter and sugar, creaming together until combined.
Add egg yolks one at a time until combined; add vanilla until combined.
In a separate bowl, combine flour, cinnamon if making oatmeal raisin cookies, brewer's yeast, salt, cream of tartar, baking soda; stir together.
Add dry ingredients to the creamed sugar and egg mixture in batches, until just combined.
Add oats and add ins.
With 1/2 tbsp scoop, make balls of cookie dough and place on prepared baking sheet (silpat or foil sprayed with cooking spray) 1" apart
Bake for 9 minutes or until edges are just browning. Once out of the oven, allow to cool on the cookie sheet for 7-10 minutes (not longer! It's the magic touch, I swear!)