1. Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius). Line muffin pans with paper liners and set aside.
2. In a medium bowl, whisk together peanut butter, oil, sugar, eggs, baking powder, baking soda, and vanilla until combined.
3. Add milk and cocoa powder. Whisk until fully combined.
4. Switch to a sturdy spoon and stir in flour. Mix until completely smooth.
5. Spoon batter into prepared muffin pans, filling each cup about two-thirds full. Set aside.