Start by preheating your oven to 350°F (175°C). Grease a standard muffin tin with cooking spray or line it with paper muffin liners for easy removal.
In a large mixing bowl, combine the all-purpose flour, granulated sugar, baking powder, and salt. Use a whisk to blend these dry ingredients together thoroughly to ensure even distribution.
In a separate bowl, mix together the melted unsalted butter, large egg, milk, and vanilla extract. Beat these ingredients until they form a smooth mixture, ensuring the butter is not too hot to cook the egg.
Pour the wet mixture into the bowl containing the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. Avoid overmixing; a few lumps are perfectly fine for a tender muffin.
Spoon a small amount of the batter into each muffin cup, filling them about one-third of the way full.
Place a generous teaspoon of Nutella in the center of each filled muffin cup. This will create a delicious surprise center! Then, top the Nutella with more muffin batter, filling each cup until they are about two-thirds full.
If using, sprinkle the chopped hazelnuts on top of each muffin for an added layer of crunch and flavor.
Place the muffin tin in the preheated oven and bake for approximately 18-20 minutes. To check for doneness, insert a toothpick into the center of a muffin; it should come out clean, although a little melted Nutella is acceptable.
Once baked, allow the muffins to cool in the tin for about 5 minutes. Then, transfer them to a wire rack to cool completely, which will help maintain their fluffy texture.
Notes
For an inviting display, arrange the muffins on a decorative plate. Consider drizzling a little melted Nutella on top for that extra decadent flair.