Preheat oven to 350°F (180°C) and grease a 12-hole muffin pan.
In a large bowl, sift together the flour and cocoa powder. Stir in the brown sugar.
Make a well in the center and add the chocolate hazelnut spread, eggs, milk, and melted butter.
Mix gently until just combined. Do not overmix.
Spoon half of the batter into the muffin cups.
Add a teaspoon of extra chocolate hazelnut spread in the center of each muffin.
Cover with remaining batter.
Bake for 20–25 minutes or until just firm to the touch.
Cool slightly in the pan, then transfer to a wire rack.