In a medium bowl, whisk together the flour, cocoa, baking soda and salt.
In a separate large bowl, beat together the butter, brown sugar and granulated sugar until fluffy.
Beat in the egg and vanilla extract.
With the mixer on a low speed, mix the dry ingredients into the butter mixture. Go slowly, as the flour can blow up.
Stir in ½ cup peanut butter chips.
Cover the bowl with plastic and refrigerate for at least 2 hours or up to 48 hours. I prefer to chill the dough for 4+ hours.
When ready to bake, preheat the oven to 350F (180C) and line cookie sheets with parchment paper or baking mats.
If the dough has chilled overnight, take it out of the fridge and let it sit on the counter for 5-15 minutes so that it can soften slightly.
Form the dough into balls with about 1 - 1.5 tablespoons of dough each. They should be about the size of a ping pong ball, or a little smaller. I typically get between 24-26 cookies in total. Place 2 inches apart on a lined cookie sheet. I recommend using a cookie scoop if you have one.
Bake 1 tray at a time in the preheated oven for 9-11 minutes, or until the tops look just set. Remove from the oven and place a few extra peanut butter chips on top of each cookie with the remaining ¼ cup.