Preheat oven to 350°F. Line an 8x8-inch metal baking pan with parchment paper, leaving a 2-inch overhang at all sides.
Place eggs, granulated sugar and vanilla in the bowl of a stand mixer fitted with the whisk attachment and beat on medium-high speed for 3-4 minutes, until creamy and lemon-coloured. You can also use a hand mixer for this.
Meanwhile, heat butter in a saucepan until completely melted and very hot (it will start crackling). Remove from heat and whisk in icing sugar, cocoa powder and salt until smooth.
When the eggs are pale and thick, add the warm cocoa mixture and beat for 30 seconds until smooth and shiny. Add the flour and mix it in. It should be smooth and glossy, like thick hot fudge. Fold in chocolate chips.
Pour the batter into your prepared pan bake for 20-22 minutes, until a toothpick inserted in the center comes out with moist crumbs attached. Make sure you do not over-bake these or they will be dry. Transfer pan to a wire rack and let brownies cool completely in the pan. Refrigerate for at least 1 hour before lifting out and cutting into squares.