In a large bowl, beat the packed brown sugar, softened butter, and creamy peanut butter with a hand mixer until creamy, about 2 minutes.
Add the eggs and vanilla extract and continue beating until fully incorporated.
On low speed, add the all-purpose flour, unsweetened cocoa powder, and baking powder and beat until the dough is evenly mixed.
Fold in the chocolate chips if they are not fully incorporated by the mixer.
Divide the dough in half and shape each half into a log on a sheet of plastic wrap; wrap tightly and chill in the refrigerator until firm, about 4 hours or overnight (or freeze 1–2 hours to speed chilling).
Preheat the oven to 350°F (175°C). Line baking sheets with parchment paper or silicone mats.
Working with one chilled dough log at a time, slice into 1/4-inch thick rounds and place them about 2 inches apart on prepared baking sheets.
Bake for 8–10 minutes, until the cookies just lose their glossy sheen. Let cool on the baking sheet for 10 minutes, then transfer to a wire rack to cool completely.
Place the chocolate chips and oil or shortening in a microwave-safe bowl and heat at 50% power, stirring every 30 seconds, until melted and smooth (about 2–3 minutes).
Dip cooled cookies into the melted chocolate and immediately sprinkle with chopped peanuts or other toppings, or put the chocolate in a sandwich bag, snip a corner, and drizzle decoratively over cookies, then add toppings.