Preheat the oven to 350F degrees. Line an 8x8 inch (20x20 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 for medium power (not high power), stirring between each interval until the chocolate and butter are melted. Alternatively, melt in a double boiler (ensuring the water doesn't touch the bottom of the bowl).
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 of of streaks of flour.
Stir in the chocolate chips.
Pour the batter into the prepared pan, and smooth the top with a rubber spatula.
Bake in the preheated oven for about 25-30 minutes, or until 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 (I cooled my brownies for 2 hours and the chocolate chips were still gooey). 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 (not a butter knife).