Arrange a rack in the middle of the oven and heat the oven to 325°F. Line an 8-inch or 9-inch square baking pan with parchment paper, then spray with cooking spray. (I used a 9-inch pan, so if you want thicker brownies, go for the 8-inch pan and add some extra time to your baking.)

Place oil into a microwave-safe medium bowl and heat in 4 to 6 (5-second) bursts until warm but not hot. Add unsweetened cocoa powder and whisk until completely smooth and combined. Let sit for 4 or 5 minutes so the cocoa can bloom. Meanwhile, prepare the dry and wet ingredients.

In a medium-sized bowl, place dry ingredients of all-purpose flour, salt, and baking powder in a small bowl and whisk to combine, set aside.

For the wet ingredients, in a large bowl or stand mixer, place eggs, powdered sugar and brown sugar together and using a mixer, beat on medium-high speed until fluffy and lighter in color, 2 1/2 to 3 minutes. Beat in 1 teaspoon vanilla extract until just combined.

Add 1/2 of the cocoa mixture to the egg mixture and beat on medium-high speed until shiny and fully combined, about 4-5 minutes.

Add the flour mixture and beat on low speed until fully combined. Add the remaining cocoa mixture and continue to beat on medium-high speed until shiny and fully combined, 1 to 2 minutes. Transfer to the baking pan and spread into an even layer.

Ready to add the Nutella, so make sure it’s drippy, so you may need to stir it really good and you can microwave for 15-20 seconds to get it really gooey so you can swirl it into the brownie batter easier. Spoon Nutella on top of the brownies and then swirl in into the batter as much as you like.

Let cool completely in the pan. Grasping the foil, lift the brownie slab out of the pan and place on a cutting board. Remove the foil and cut the slab into 9-16 squares.