Mix flaxseed with water in small bowl and let soak for 5 minutes.
Beat butter, white sugar, and brown sugar together in a large mixing bowl until creamy. Add egg yolks and vanilla extract; beat to incorporate. Stir flax seed mixture into the butter mixture.
Mix flour, brewer's yeast, wheat germ, baking soda, salt, and cream of tartar in a separate bowl; add to butter mixture and stir until just combined. Fold oats and chocolate chips into the dough.
Roll dough into walnut-sized balls and place 2 inches apart onto a baking sheet.
Bake in the preheated oven until the edges are golden, 10 to 12 minutes. Allow the cookies to cool on the baking sheet for 1 minute before removing to a wire rack to cool completely.