Preheat oven to 350°F (175°C). Line a 12‑cup muffin pan with liners or lightly grease.

In a large mixing bowl, whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt.

In another bowl, beat milk, oil, egg, and vanilla until smooth.

Pour wet ingredients into dry and stir until just combined—do not overmix. If desired, gently fold in chocolate chips.

Spoon batter evenly into muffin cups, filling about ⅔ full.

Drop about 1 tsp of peanut butter on top of each muffin. Using a toothpick or knife, swirl it lightly through the batter to create a marbled effect.

Bake 18–22 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.

Allow muffins to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before serving.