Preheat your oven to 350°F (175°C). This step ensures that your doughnuts bake evenly and rise perfectly.
Lightly grease your doughnut pan with non-stick spray or butter to prevent sticking. This will help your doughnuts come out easily after baking.
In a large mixing bowl, whisk together the all-purpose flour, cocoa powder, baking powder, and salt. This creates a uniform dry mixture that will ensure even flavor throughout your doughnuts.
In another bowl, beat together the granulated sugar, brown sugar, and softened butter until light and fluffy. This process helps to incorporate air into the mixture, giving your doughnuts a lovely texture.
Once the sugars and butter are creamed, add the Nutella, egg, vanilla extract, and milk. Mix until everything is well combined. The Nutella adds a wonderful flavor and moisture to the doughnuts.
Gradually add the dry mixture to the wet mixture, stirring gently until just combined. Be careful not to overmix; a few lumps are perfectly fine.
Spoon the batter into the prepared doughnut pan, filling each cavity about 2/3 full. This allows space for the doughnuts to rise without overflowing.
Bake in the preheated oven for 10-12 minutes, or until a toothpick inserted into the center comes out clean. The doughnuts should be slightly golden and spring back when touched.
Let the doughnuts cool in the pan for about 5 minutes before transferring them to a cooling rack to cool completely. This helps maintain their soft texture.
In a small bowl, whisk together the powdered sugar, 2 1/2 Tbsp Nutella, 4-6 tsp milk, and 1/2 tsp vanilla extract until smooth. Adjust the milk to reach your desired consistency.
Once the doughnuts are completely cool, dip the tops into the Nutella glaze, allowing any excess to drip off. For an extra touch, sprinkle with chopped, toasted hazelnuts or colorful sprinkles.