Preheat oven to 350° Spray 3 smaller 6-inch cast iron skillets, or a 10-inch cast iron skillet with non-stick cooking spray.
Prepare Topping:
Using a stand mixer or a hand held mixer and a medium bowl, mix the 2 tablespoons softened butter and 1 tablespoon peanut butter on medium speed until combined. Mix in flour and brown sugar, and mix until crumbly. Stir in the peanuts and set aside.
Prepare Cake:
In the bowl of an electric stand mixer or using a hand mixer and a large bowl, beat ½ cup butter, ½ cup 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, baking powder, and salt together. Gradually add flour mixture to butter mixture alternating with milk, starting and ending with the flour mixture. Scrape the sides of the bowl down occasionally as you mix the ingredients in.
Pour half of the batter in the skillet, and spread with an offset spatula. Spoon Nutella over the top of the batter, and spread with an offset spatula. Top with remaining batter, smoothing the top with the spatula. Sprinkle the topping over the batter.
Bake until a wooden toothpick inserted in the center comes out clean, 23-25 minutes for the 6-inch skillets or approximately 40 minutes for the 10-inch skillet. Loosely cover with foil to prevent excess browning if necessary.