Preheat the oven to 350°F / 175°C.
Place the ground flax and water into a medium sized mixing bowl. Add the water, stir, and set this mixture aside.
In a separate, medium small mixing bowl, whisk together the flour, baking powder, and salt.
Add the sugar, peanut butter, and vanilla extract to the flax mixture. Whisk until this mixture is smooth and evenly combined.
Add the flour mixture to the wet ingredients. Beat with a spoon until you have an evenly combined cookie dough. Allow this mixture to rest in the fridge for 15-30 minutes, or you can cover the bowl and refrigerate it for up to 2 hours.
Scoop the dough up by heaping tablespoons and use your hands to roll it into balls that are about 1 1/4-inches / 3cm in diameter. You’re aiming for 10-12 cookies.
Arrange these cookies onto a nonstick or parchment or silpat-lined baking sheet. Gently press a small amount of the chopped peanuts on top of each cookie, using light pressure. Then, sprinkle the cookies with the Maldon.
Bake the cookies for 9 minutes. The should be puffy and just golden—the texture is meant to be soft, so be very careful not to overbake!
Transfer the cookies to a wire cooling rack and cool for at least 20 minutes before enjoying. Store the cookies at room temperature in an airtight container for up to 4 days.