Preheat your oven to 350 degrees Fahrenheit. This step is crucial for ensuring even baking. A well-preheated oven will help the brownies rise perfectly and achieve that desirable texture.
In a small saucepan, melt the butter and 1/2 cup of the chocolate chips over medium-low heat. Stir frequently until completely melted. The key is to avoid burning the chocolate, so keep an eye on it! Once melted, remove the saucepan from heat and stir in Nutella until well combined.
Next, add sugar to the mixture and beat well. This will help incorporate air into the batter, making it lighter. You want to ensure that the sugar is fully mixed in before moving on to the next step.
Add in the eggs and yolk, mixing until well combined. The eggs are essential for creating that rich texture, so make sure they are at room temperature for better incorporation.
Now, add the flour and stir until just combined. Be cautious not to over-mix the batter; doing so can lead to cake-like brownies instead of the fudgy ones you desire.
Fold in the remaining chocolate chips gently. This is where the brownies will get their gooey chocolate bursts, so take care to distribute them evenly without overworking the batter.
Pour the batter into a lightly greased 9x13 baking dish. Use a spatula to spread it evenly, ensuring the corners are filled.
Bake in the preheated oven for 25 to 30 minutes. Keep an eye on them, as baking times can vary based on your oven.
Once baked, remove the brownies from the oven and let them cool for at least 30 minutes. Patience will be rewarded with chewier brownies!
After cooling, cut into squares and serve. If you’d like to add a special touch, slather additional Nutella on top and sprinkle with chopped pistachios right before serving.