Preheat the oven to 350ºF (175ºC). Grease a 9x3-inch baking dish with butter to ensure the brownies don’t stick. Set aside.
In a saucepan over low heat, melt the butter completely. Be sure to keep the heat low to prevent the butter from burning.
Add the white sugar and brown sugar to the melted butter and stir continuously. Let the mixture cook on low heat for about 2 minutes, making sure it doesn’t burn or boil. Remove the pan from the heat, add the Nutella, and stir until the mixture is smooth and fully combined.
Transfer the butter, sugar, and Nutella mixture to a large bowl. Add the eggs, cocoa powder, salt, espresso powder, vanilla extract, and baking powder. Using a mixer (handheld or stand), beat the ingredients until everything is well incorporated and smooth. If you don’t have a mixer, you can whisk by hand—it just takes a little more effort!
Transfer the butter, sugar, and Nutella mixture to a large bowl. Add the eggs, cocoa powder, salt, espresso powder, vanilla extract, and baking powder. Using a mixer (handheld or stand), beat the ingredients until everything is well incorporated and smooth. If you don’t have a mixer, you can whisk by hand—it just takes a little more effort!
Pour the batter into the prepared buttered baking dish and spread it out evenly. The batter will be thick and sticky—don’t worry, that’s completely normal! Use a spatula to gently smooth the surface.Melt a small amount of Nutella in the microwave for about 30 seconds. Using a spoon, drop dollops of Nutella evenly over the top of the brownie batter, arranging them in about 3 rows of 4.With a knife, gently drag through the brownie batter, pulling through the Nutella dollops to create a swirl pattern. Be careful not to overdo it—you want distinct swirls throughout the batter. Bake the brownies for 28 to 35 minutes, or until a toothpick inserted near the edges comes out mostly clean. The edges should be fully baked, while the center should look moist but not uncooked. Remember, the brownies will continue to cook slightly as they cool!