Preheat oven to 350 degrees. Spray a muffin pan with nonstick cooking spray, or put in muffin liners.
Combine flour, brown sugar, baking powder, baking soda, and salt in a bowl. Stir.
Add in yogurt, buttermilk, eggs, peanut butter, and vanilla. Stir until everything is mixed in.
Divide thick batter among the 12 muffin spots. They'll be about 2/3 full.
Place 1 tablespoon of Nutella on top of each. Use a toothpick to swirl the Nutella around. Make sure you do it all the way to the bottom so there is Nutella throughout.
Bake in the preheated oven for about 18-20 minutes.