About 2/3 cup Bittersweet Chocolate, finely chopped
1 tablespoon Light Corn Syrup
1/4 teaspoon Flake Sea Salt, preferably Maldon
Position an oven rack in the center of the oven and preheat the oven to 180ºC. Line the bottom of the pan with parchment paper, and then spray the pan generously with coconut oil baking spray. Set aside.
Place the butter and brown sugar in the bowl of your KitchenAid Stand Mixer fitted with the flat beater. Add the caster sugar and turn the mixer to low speed, open the ingredient valve, and sift the sugar into the mixer bowl. Cream the mixture on medium-high speed until fluffy, about 3 minutes. Scrape down the sides of the bowl. With the mixer running on medium, add the eggs, one at a time, incorporating each before adding the next. Beat in the sour cream and vanilla.
Add the flour, cocoa, and salt. With the mixer on medium-low speed, sift in the flour mixture. Be careful not to overmix the batter. Remove the mixer bowl, and use a rubber spatula to blend in any flour or coca remaining on the sides of the bowl. Scrape the batter in the prepared pan and spread evenly. Bake until a toothpick inserted into the center of the cake comes out with a few moist crumbs clinging to it, 45 to 55 minutes. (These are very decadent fudgy brownies, and not meant to be cake-like.) Transfer to a rack and cool completely.
When the brownies are almost cool, make the chocolate ganache. In a medium metal or heatproof bowl, combine the cream, chocolate, corn syrup, and salt. Put the bowl over a pan of barely simmering water and heat, stirring with a heatproof spatula, until the chocolate mixture is smooth and shiny. Remove from the heat. Using an offset spatula or small rubber spatula, spread the ganache evenly over the brownies. Set aside until set.
To serve, cut the brownies into 5-cm squares (or larger if desired!), and use a spatula to transfer them to individual dessert plates. Garnish each brownie with a sprinkling of flakey sea salt.
Tightly wrap any remaining brownies and place in an airtight container. They will keep at room temperature for 3 to 4 days.