Line a 12-cup muffin tin with paper liners, or grease the cups well.
Preheat your oven to 180 °C conventional or 160 °C convection.
To make the batter, weigh all the dry ingredients into a bowl and whisk to combine: flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Weigh all the wet ingredients—sour cream, oil, milk, eggs, and hazelnut spread—directly into the mixing bowl. Mix for 15 sec / speed 4.
Next, add the dry ingredients and mix briefly but thoroughly for 10 sec / speed 4.
Finally, add 100 g of the chocolate chunks and mix into the batter for 4 sec / speed 4 / reverse.
Divide the batter evenly among the prepared muffin cups using a piping bag or two teaspoons.
Sprinkle the remaining chocolate chunks on top and bake on the lower rack for 20-25 minutes.
Towards the end of the baking time, test for doneness with a toothpick. Insert a wooden toothpick into the center of a muffin: if it comes out with wet batter attached, they need a bit more time. If it comes out clean, your muffins are perfectly baked!
Let the muffins cool in the tin for about 5 minutes, then remove them and let them finish cooling on a wire rack (covered lightly to keep them moist).
Notes
If you like, dust the muffins with a little powdered sugar right before serving.
Feel free to swap out the dark chocolate chunks for caramel bits or white chocolate chips!
Berries like cherries, blueberries, or raspberries are delicious additions, too. Just gently fold them into the batter by hand so they stay whole.