Preheat the oven to 180C/350F/Gas 4. Line an 8x8-inch square baking pan with foil and spray with baking spray. Set aside.
Place the butter, sugar, cocoa powder, and salt into a microwave-safe bowl, and microwave in 1 minute intervals, stirring after each one, until the butter is melted and the mixture feels very warm to the touch.
Mix in the eggs, one at a time. Fold in the flour until fully combined. Fold in the chocolate chips (they will melt into the batter, but it's fine! It helps produce a shiny top). Stir in the vanilla extract.
Pour the batter into the prepared pan, and spread out evenly. Bake for 30 - 35 minutes, or until a toothpick inserted into the centre comes out almost clean with a few moist crumbs.
Allow to cool completely in the pan on a wire rack.
For the Nutella Fudge
Place the condensed milk, chocolate chips, and Nutella in a heat-proof bowl over a pan of simmering water. Stir until melted and smooth, then spread evenly over the brownies.
Chill in the fridge until set, then cut into squares and enjoy!
Notes
Brownies stay fresh, stored in an airtight container, in the fridge for up to 1 week. They also freeze well for up to 3 months. Thaw overnight in the fridge before serving.