Melt the butter and Nutella in a microwave-safe bowl or saucepan over low heat, stirring until smooth, about 2-3 minutes. Allow to cool slightly.

Whisk together the eggs and sugar in a large mixing bowl for about 1-2 minutes until light and fluffy.

Pour the cooled Nutella and butter mixture into the egg-sugar blend along with the vanilla extract. Stir gently until fully combined.

Sift together the flour, cocoa powder, and salt in a separate bowl. Fold this into the wet ingredients until no visible flour remains.

Gently stir in the chocolate chips or chopped chocolate into the batter.

Line an 8×8-inch baking pan with parchment paper, pour in the brownie batter, and smooth the top. Swirl in extra Nutella if desired.

Preheat oven to 350°F (175°C) and bake for 28-32 minutes. A toothpick should come out with a few moist crumbs.

Cool completely in the pan for 20-30 minutes, then lift out using parchment to cool fully before slicing.