Preheat the oven to 400 degrees F. Line a 12-cup standard muffin tin with paper liners; set aside.
In a medium bowl, combine the flour, sugar, cinnamon, baking powder and salt.
In a large glass measuring cup or another bowl, whisk together the milk, vegetable oil, egg and vanilla.
Pour mixture over dry ingredients and stir using a rubber spatula just until moist.
Fill muffin liners 3/4 full with batter. Add one heaping teaspoon of Nutella to the top of each one. Using the measuring spoon, dip the spoon into the batter then gently push the Nutella just below the surface. Swirl the batter around a bit to cover the Nutella.
To make the topping, combine the cocoa and sugar and stir together well. Generously sprinkle each muffin with the cocoa topping until no batter is visible.
Place into oven and bake for 15-18 minutes. Remove from oven and let cool in paint for five minutes. Gently lift each muffin with a spoon and tap once to knock off the excess cocoa sugar.
Place muffins on on a wire rack to cool. Can be eaten warm or at room temperature. Caution, do not eat hot as the Nutella needs a little time to cool down.