• Set oven and prep pan. Heat the oven to 350°F and line an 8 x 8-inch baking pan with parchment paper.
• Make batter. Add the Nutella, cocoa powder, baking powder and eggs to a medium-sized mixing bowl. Use an electric hand mixer to blend everything together until smooth.
• Bake. Pour the batter into the prepared baking pan and spread it evenly. Bake in the preheated 350°F oven until the edges begin pulling away from the sides and it's no longer wiggly in the center, about 25 minutes.
• Cool and slice. Place the pan on a cooling rack and let it sit for at least 30 minutes. Then carefully lift it out of the pan with the parchment paper. Place it on a cutting surface and slice it into about 25 squares. (If you're adding the hearts, it's a bit easier if you slice 16 instead, so they're a bit bigger.)

For the hearts

• Use a tiny heart-shaped cookie cutter, about ¾-inch at the widest part, to cut out a small heart on top of each brownie. DO NOT push the cutter all the way to the bottom of the brownie -- about half way is perfect. Once the cutter is in the brownie, gently pull it up. This will pull out a bit of the brownie in a little heart shape, and will only work if the brownies are not too warm. (Save these cute brownie hearts and serve as part of this treat!)
• Add the ¼ cup of Nutella to a small zip-lock bag and cut a tiny bit off of one of its bottom corners. Then, holding the open tip close to the bottom of each hollowed-out heart, squeeze gently to fill it up. You can use a toothpick or the tip of a knife to spread it evenly if necessary.