Preheat oven to 350 degrees F. Line 12 cups of a mini muffin pan with liners or grease with non-stick cooking spray.

Put the Nutella and egg in a medium bowl and whisk until smooth and well blended. Add the flour and whisk until blended.

Spoon the batter into the prepared muffin tins (about 2/3 full). Melt peanut butter slightly in the microwave, about 20 seconds. Place about a 1/2 teaspoon of peanut butter on each muffin and swirl into the batter with a toothpick. Sprinkle with chopped peanuts.

Bake until a pick comes out with wet, gooey crumbs, 11 to 12 minutes. Don't overbake! Set on a rack to cool completely. Serve immediately or cover and store at room temperature for up to 3 days.