Scoop the Nutella into teaspoon portions onto a parchment lined baking tray. Chill the freezer until ready to use.
In a large bowl, whisk together the flour, baking powder, baking soda, and salt.
In a separate large mixing bowl, whisk together the buttermilk, mashed banana, melted butter, sugar, eggs, sour cream, oil, and vanilla extract.
Whisk the dries into the wets.
Line a 12-cup muffin tray with paper liners.
Fill the liners with about 3-4 tbsp of batter (filling the liner about half full).
Then press in one of the frozen nutella filling into the center of each one.
Scoop another 3-4 tbsp of the batter on top. The batter should be fuller than the top of the liner. (see post for visual)
Warm the rest of the nutella in the microwave for 20 seconds until it runs smoothly off the spoon.
Spoon about 1 tsp of the nutella on top of each muffin. Then use a toothpick, skewer, or butter knife to gently swirl the chocolate and batter together.
Bake for 8 minutes at 425 F, then decrease the heat to 350 and continue baking for another 12-14 minutes or until the tops bounce back when touched.
Notes
*Measure your dry ingredients properly. This is my #1 baking tip! Do not ever scoop a measuring cup into your flour, as this always leads to using too much flour. Instead, use the spoon-level method. This means fluffing the flour first, then spooning it into your measuring cup/spoon. For the BEST results, use a kitchen scale!*