Preheat the Oven and Prepare the Muffin Tin:

Preheat your oven to 375°F (190°C). Line a muffin tin with paper liners or lightly grease with non-stick spray.

Prepare the Peanut Butter Swirl:

In a small bowl, mix the creamy peanut butter, powdered sugar, melted butter, and vanilla extract until smooth and well combined. Set aside.

Mix the Dry Ingredients:

In a large bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and salt. Set aside.

Cream the Butter and Sugars:

In another bowl, using an electric mixer, beat the softened butter, granulated sugar, and brown sugar together until light and fluffy, about 2-3 minutes.

Add the Eggs and Vanilla:

Beat in the eggs one at a time, followed by the vanilla extract, until fully incorporated.

Combine the Wet and Dry Ingredients:

Gradually add the dry ingredients to the wet mixture, alternating with the milk and sour cream (or Greek yogurt). Begin and end with the dry ingredients, mixing until just combined. Be careful not to overmix.

Fill the Muffin Cups:

Spoon the muffin batter into the prepared muffin cups, filling each about 2/3 full.

Add the Peanut Butter Swirl:

Drop a teaspoon of the peanut butter mixture onto each muffin. Use a toothpick or skewer to swirl the peanut butter into the batter, creating a marbled effect.

Bake the Muffins:

Bake in the preheated oven for 18-22 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.