Preheat the oven to 350ºF.
Mix together the flax and water and let sit until slightly thickened, about 5-7 minutes.
Cream together the butter, peanut butter, and sugars until light and fluffy, about 5 minutes.
Add the flax and vanilla.
Sift together the dry ingredients (flour, baking soda, and salt) and add to the mixing bowl. Mix until just combined.
Scoop the dough onto a parchment-lined baking sheet. Press the top of the cookies down with a fork twice in a criss-cross pattern, then sprinkle with granulated sugar and a small pinch of salt.
Bake 20-25 minutes until golden brown.