Step 1: Preheat the Oven - Preheat your oven to 350°F (175°C).
Step 2: Prepare the Cupcake Pan - Line a cupcake pan with cupcake liners.
Step 3: Melt the Butter - In a medium saucepan over low heat, melt the butter completely. Remove from heat and let it cool slightly.
Step 4: Mix Wet Ingredients - In a large mixing bowl, combine the melted butter, granulated sugar, and vanilla extract. Whisk together until well blended. Add the eggs one at a time, mixing well after each addition.
Step 5: Combine Dry Ingredients - In a separate bowl, whisk together the cocoa powder, flour, baking powder, and salt. Gradually add this mixture to the wet ingredients, stirring until just combined.
Step 6: Fold in Peanut Butter Chips - Gently fold in 1 1/3 cups of peanut butter chips into the batter, reserving the remaining 1/3 cup for topping the cupcakes.
Step 7: Fill the Cupcake Liners - Spoon the batter evenly into the prepared cupcake liners, filling each about two-thirds full. Sprinkle the reserved peanut butter chips on top of each cupcake.
Step 8: Bake - Place the cupcake pan in the preheated oven and bake for 18-20 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Step 9: Cool - Once baked, remove the cupcakes from the oven and let them cool in the pan for about 5 minutes. Then, transfer them to a cooling rack to cool completely.