Preheat the oven to 200 degrees C (fan-assisted) and line a muffin tin with cake cases.
Toss together the flour, cocoa and bicarb, until well mixed.
In another bowl whisk together the eggs, buttermilk, oil, and sugar for a few minutes.
Pour these wet ingredients into the dry ingredients and mix together until the batter is smooth (around 2 minutes with an electric whisk) but don’t overmix, a few lumps are fine.
Gently fold through the chocolate chips.
Spoon 2 tbsp of muffin mix into each cupcake case.
Add a big teaspoon of Nutella.
Top with another tablespoon of muffin mix.
Bake at 200 degrees for 10 minutes, before reducing the temperature to 160 for 10-15 minutes.
Notes
If you don’t have buttermilk, add 2tbsp lemon juice to normal milk and leave it on the side until it’s at room temp.
Muffins are ready when the tops are domed and still bounce back a little when pressed. You can also use a skewer to check that the centre is cooked, trying your best to avoid the Nutella middle.