Preheat the oven to 350F (180C) degrees or 325F (170C) for a convection oven. Line an 8x8 inch (20x20 cm) pan with parchment paper, leaving an overhang around the edges.
Whisk together the flour, cocoa, cornstarch and salt in a small bowl. Set aside.
In a separate large bowl, melt together the butter and chopped chocolate. Melt in the microwave using 45-second intervals on medium power and stir between each interval. Alternatively, melt using a double boiler over low-medium heat so that the water in the saucepan is gently simmering. Ensure that the simmering water in the saucepan does not touch the bottom of the bowl with the chocolate and butter.
Once melted, let the chocolate and butter mixture sit for a few minutes so that it is not longer hot (warm is OK). Then whisk in the sugar, eggs plus additional egg yolk, orange zest and vanilla extract until the mixture is glossy and you no longer see pieces of egg (you will still see sugar granules).
Sift the dry ingredients into the butter and chocolate mixture. Then whisk tother until you no longer see streaks or clumps of dry ingredients in th emixture.
Gently fold in the chopped orange chocolate using a rubber spatula or wooden spoon.
Spoon the batter into the prepared pan and smooth the top.
Bake in the preheated oven for about 28-33 minutes, or until an inserted toothpick comes out with a few damp brownie crumbs. If it comes out with gooey brownie batter, the brownies aren't done baking yet.
Cool the brownies in the pan for at least 90 minutes. For clean cuts, leave them in the pan until it is not longer warm to the touch (about 3 hours). When ready to slice, lift the brownies out of the pan using the overhang of the parchment paper. Then place on a cutting board and slice.