Preheat oven to 350°F. Spray 3-6" cast iron skillets or a 10" skillet with non-stick cooking spray.
In a medium bowl, whisk together flour and brown sugar. Using a mixer on medium speed, beat butter and peanut butter until crumbly. Stir in the peanuts and set aside.
In the bowl of an electric mixer, beat butter, peanut butter and sugar on medium speed until fluffy. Add eggs one at a time, beating well after each addition. Beat in sour cream and vanilla.
In a medium bowl whisk flour, salt, and baking powder together. Gradually add flour mixture to butter mixture alternating with milk, starting and ending with the flour mixture.
Pour half of the batter in the skillet. Spoon Nutella over batter. Top with remaining batter. Sprinkle with topping.
Bake until a wooden toothpick inserted in the center comes out clean, 23-25 minutes for 6" skillets or approximately 40 minutes for a 10-inch skillet. Loosely cover with foil to prevent excess browning if necessary.
This recipe was calculated using the exact brands and measurements I used to make this recipe. If you are following a strict diet please note changing anything will cause the nutritional info to change. My calculations are intended as a guide only.