Prepare the oven and mold: Preheat your oven to 160 °C (325 °F) and line a brownie pan with wax paper or grease it well.
Melt and mix: In a heat-proof bowl, combine the unsalted butter, sugar, cocoa powder, and salt. Place the bowl over a pot of simmering water (a water bath) and stir until the butter is melted and everything is well mixed. Once done, let it cool for a couple of minutes.
Add eggs and vanilla: Stir in the vanilla extract. Incorporate the cold eggs one at a time, whisking vigorously after each addition until fully blended.
Integrate flour and nuts: Sift the all-purpose flour into the mixture and mix until you achieve a smooth batter. If using, gently fold in the chopped nuts.
Nutella and bake: Pour the brownie batter into the prepared baking pan. Swirl hot Nutella on top with a knife for a marbled effect. Bake for 20-25 minutes or until a toothpick inserted comes out with some moist crumbs.
Cool and decorate: Allow the brownies to cool completely before cutting them into squares. Top with more melted Nutella or sprinkle with Kinder pieces for an extra touch.