Preheat the oven to 350°F (175°C). Line an 8×8 inch (20×20 cm) baking pan with parchment paper or aluminum foil so that there’s an overhang around the edges.
Chop the chocolate into small pieces. Add the chopped chocolate and butter to a large, heatproof bowl. Microwave for 45-second intervals at medium power, stirring between each interval until the chocolate and butter are melted. Alternatively, melt in a double boiler.
Whisk the butter and dark chocolate together until combined. Then whisk in the sugar, vanilla extract, 2 eggs, and additional egg yolk.
Sift in the flour, cocoa, and salt. Then gently whisk the batter together until you no longer see lumps or streaks of flour.
Stir in the chocolate chips.
Pour the batter into the prepared pan and smooth the top with a rubber spatula.
the tops look set and an inserted toothpick comes out with a few moist crumbs.
Cool the brownies in the pan for at least 2 hours. Lift the brownies out of the pan using the overhang of the parchment paper/aluminum foil and place on a cutting board. Slice with a sharp knife.