Line a small plate or baking sheet with parchment paper. Transfer the Nutella to a zip-top bag or piping bag and pipe 10 logs, each about 1.5 inches long, onto the parchment. Freeze until ready to use.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a large bowl, whisk together the milk, heavy cream, sugar, melted butter, eggs, sour cream, oil, and vanilla until smooth and fully combined.
Add the dry ingredients to the wet ingredients and whisk until only a few streaks of flour remain. Switch to a rubber spatula and fold in the chocolate chips until evenly distributed and no dry flour remains.
Cover the bowl with plastic wrap and let the batter rest at room temperature for 1 hour.
Meanwhile, preheat the oven to 400°F (205°C). Line a 12-cup metal muffin pan with tulip liners. For tall, bakery-style muffins, line only every other cup (6 liners total) and bake the batter in two batches.
Once the batter has rested, divide it evenly between the prepared liners (about 130 g each), filling each one to just below where the tulip points begin.
Insert one frozen Nutella log into the center of each muffin until completely submerged, using your finger to spread some of the muffin batter on top to cover it if needed. Work quickly, as the Nutella softens fast. Return the remaining Nutella to the freezer until ready to use.
For swirled Nutella tops, you can use the same piping bag to pipe a squiggle on top and use a toothpick to swirl it into the batter. You can also spoon it on top if that's easier. Sprinkle with additional mini chocolate chips, if desired.
Bake for 8 minutes. Without opening the oven door, reduce the oven temperature to 350°F (180°C) and continue baking for 15–20 minutes, or until the muffin tops are golden brown and the tops are no longer wet or sticky. The centers should still feel slightly soft when gently pressed.
Cool the muffins in the pan for 5 minutes, then carefully transfer them to a wire rack to cool completely.
Allow the pan to cool completely before lining it again and baking the remaining batter.
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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!*Storage: Keep leftover muffins in an airtight container at room temperature for up to 4 days. For longer storage, wrap each muffin individually in plastic wrap. Keep in an airtight container or freezer bag for up to 3 months.