To create these amazing Chocolate Peanut Butter Swirl Muffins, follow these simple steps:

1. Preheat Oven: Begin by preheating your oven to 350°F (175°C). Line a muffin tin with paper liners or lightly grease it.
2. Mix Dry Ingredients: In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, and salt until well combined.
3. Combine Wet Ingredients: In another bowl, mix the milk, vegetable oil, eggs, and vanilla extract. Whisk until smooth.
4. Combine Mixtures: Gradually add the wet mixture to the dry ingredients, stirring until just combined. Do not overmix; a few lumps are okay.
5. Prepare Peanut Butter Swirl: In a small microwave-safe bowl, warm the peanut butter slightly (about 20-30 seconds) to make it easier to swirl.
6. Add Chocolate Chips: If using, fold in the semi-sweet chocolate chips into the batter.
7. Fill Muffin Cups: Spoon the batter into the prepared muffin tin, filling each cup about halfway.
8. Swirl in Peanut Butter: Add a teaspoon of warmed peanut butter on top of each muffin cup. Use a toothpick or knife to create a swirl pattern.
9. Bake: Place the muffin tin in the preheated oven and bake for 20-25 minutes or until a toothpick inserted in the center comes out clean.
10. Cool: Once baked, remove from the oven and let the muffins cool in the tin for about 10 minutes before transferring them to a wire rack.

Following these steps will ensure your muffins come out perfectly every time!