Preheat oven to 350 degrees F (175 degrees C).
In a small bowl, combine All-Purpose Flour (1 cup), Unsweetened Cocoa Powder (1/4 cup), Baking Soda (1 teaspoon), and Salt (1/4 teaspoon). Set aside.
In a large mixing bowl, beat together Peanut Butter (1 cup), Brown Sugar (3/4 cup), Caster Sugar (1/4 cup), Butter (1/2 cup), Large Egg (1), and Vanilla Extract (1/2 teaspoon) with a mixer until creamy.
Gradually mix in the flour and cocoa mixture. Beat until the cookie dough is moist and comes together.
Using a tablespoon or teaspoon depending on how large you want your cookies, scoop out even sized amounts. Form into a ball and place onto a cookie sheet 1 1/2 inches apart.
Mash slightly with a fork to flatten the cookies.
Bake for 7-8 minutes, then remove from the oven.
After 1 minute, transfer the cookies to a wire rack to cool.