In a medium bowl, whisk together the melted butter and oil. Add in the sugar.
Whisk in eggs, vanilla extract, and milk until combined.
Slowly add in the flour, salt, baking soda, and baking powder and continue to mix together until just combined (do not over-mix! It's okay if there are some lumps in the dough). Stir in the mini chocolate chips.
Let the muffin batter rest in the fridge for 30-60 minutes (this creates taller muffins).
Preheat the oven to 425°F and line a muffin tin with 12 liners.
Add a little bit of muffin muffin batter to the bottom of each muffin liner (around 1½ Tablespoons). Add one scant Tablespoon of Nutella on top, trying to keep it in the center and away from the edges.
Add more muffin batter on top of the nutella, filling the muffin tins all the way to the top of the liners. Repeat with the remaining muffin liners until all batter is gone.
Melt 3 Tablespoons of chocolate hazelnut spread in a small microwave safe bowl until it's runny. Drizzle about ½ teaspoon on top of each muffin and use a toothpick to gently swirl it around the top.
Bake the muffins for 5 minutes at 425°F. After 5 minutes, leave the muffins in the oven and lower the oven temperature to 350°F. Bake for an additional 7-10 minutes, or until a toothpick inserted in the centre of the muffins comes out clean.
Let the muffins cool or a cooling rack. The chocolate hazelnut spread will not be as melted and soft the next day, but you can microwave the muffins for a few seconds before serving to recreate the fresh out of the oven texture.