With the rack in the middle position, preheat the oven to 325°F (165°C). Butter an 8-inch (20 cm) square pan and line with a strip of parchment paper, letting it hang over two sides.
In a bowl, over a double boiler or in a microwave oven, melt the chocolate with the butter. Let cool.
In another bowl, whisk together the eggs, sugar and brown sugar until smooth. Add the melted chocolate, then the flour. Mix to combine. Spread the batter out in the prepared pan.
Bake for 30 minutes or until a toothpick inserted in the centre comes out with a few crumbs attached (the toothpick should not be completely clean). Let cool completely in the pan, about 2 hours.
Dark Chocolate Ganache
Meanwhile, place the chocolate in a bowl.
In a small pot, bring the cream and corn syrup to a boil. Pour over the chocolate and let melt for 2 minutes without stirring. Mix with a spatula until shiny and smooth. Add the butter. Let the ganache cool at room temperature until thick and spreadable, 1 to 2 hours.
Spread the ganache evenly over the cooled brownies. Cut into squares.