Sift flour, cocoa, baking soda and salt into a medium bowl and whisk to blend evenly.

Beat butter with both sugars in the bowl of a stand mixer fitted with the paddle attachment or using a hand mixer until smooth and slightly fluffy. Beat in egg and vanilla until well combined.

Add flour mixture and mix on low speed until mostly combined but a few streaks of flour remain. Add chocolate and peanut butter wafers and continue to mix on low just enough to distribute them through the dough. Wrap dough well in plastic wrap and refrigerate for at least 2 hours or over night. (Note: you can also bake these cookies right away without refrigeration, but in that case I would increase flour to 1 ⅓ cups to reduce spread).

Preheat oven to 350°F. Line two large baking sheets with parchment paper. If left overnight, let the cookie dough stand at room temperature for 15 minutes to soften.

Roll about 2 tablespoons of dough into balls and place onto prepared baking sheets, spacing them at least 2 inches apart. Flatten with the palm of your hand and bake for 9-11 minutes. Remove from oven and let cookies cool on trays for 2-3 minutes, then transfer to a wire rack to finish cooling.