Preheat oven to 350ºF.
Mix together the flax and water and let sit until slightly thickened, about 5-7 minutes.
Cream together the butter and sugar until smooth. Add the vanilla and flax.
Sift together the dry ingredients (flour, salt, and baking powder), then fold into the wet mixture.
Fold in the mix-ins.
Pour into a parchment-lined 8"x8" pan and spread evenly. Sprinkle with a handful of chocolate chips and a big pinch of salt.
Bake 35-45 minutes until golden brown. I like to underbake them a little bit so they have the brownie/underdone cookie texture. Yummm.