In a medium bowl, whisk the buttermilk, granulated sugar, vegetable oil, melted butter, brown sugar, eggs and vanilla extract until smooth and well combined.
Fold the dry ingredients into the wets. Scoop 3 tablespoons of batter into each muffin cup. Press a piece of frozen Nutella into the center of each then top with more batter.
Heat up the remaining Nutella in the microwave for 15 seconds or until runny then scoop about 1 teaspoon on top of each muffin and swirl with a toothpick. Bake and enjoy!