Preheat the oven: Set to 350°F (175°C). Line a mini muffin pan with paper liners or lightly grease with cooking spray. For regular muffin pans, grease well.

Mix the wet ingredients: In a medium bowl, whisk the eggs until well combined. This helps create a shiny top and even texture.

Add the chocolate-hazelnut spread: Stir the spread into the eggs until smooth and glossy. The mixture should look uniform with no streaks.

Whisk in dry ingredients: Sift or sprinkle in the flour and cocoa powder. Gently fold with a spatula until just combined. Do not overmix—stop when you no longer see dry spots.

Portion the batter: Spoon the batter into the prepared pan, filling each cup about 3/4 full. If using, sprinkle a few mini chocolate chips or a pinch of flaky salt on top.

Bake: For mini muffin pans, bake 10–12 minutes. For standard muffin pans, bake 14–16 minutes. The tops should be set and slightly crackly, and a toothpick should come out with a few moist crumbs.

Cool: Let the brownie bites cool in the pan for 5 minutes, then transfer to a wire rack. They will set as they cool.

Serve: Enjoy warm for the fudgiest texture, or at room temperature for a slightly chewier bite.