Preheat oven to 350°F. Line a 9x9-inch square baking pan with a parchment paper sling.
Combine finely chopped chocolate and butter in a large microwave-safe bowl. Melt in the microwave using 30 second bursts of heat, stirring between each interval, until smooth.
4 ounces bittersweet chocolate, 3/4 cup salted butter
Whisk in cocoa powder and sugars until combined. Add eggs and vanilla extract. Whisk or beat well until completely combined.
1/2 cup unsweetened cocoa powder, 1 cup granulated sugar, 1/2 cup light brown sugar, 3 large eggs, 2 teaspoons vanilla extract
Stir in flour and salt just until no streaks of flour remain. Pour brownie batter into prepared pan and bake to 30-35 minutes until a toothpick inserted in the center comes out with just a few crumbs. Set aside to cool.
3/4 cup all-purpose flour, 1/2 teaspoon salt
Meanwhile, add granulated sugar for the caramel to a medium saucepan and cook sugar over medium heat, stirring occasionally with a wooden spoon, just until melted and golden brown. At first it will look like the sugar is just clumping together, but eventually it will melt. Be careful not to burn the sugar.
1 1/2 cups granulated sugar
Carefully stir in the cream, butter and vanilla until the butter has melted. Continue to cook and stir the caramel until it reaches 240°F on a candy thermometer. This should take another 3-5 minutes.
1 cup heavy cream, 1/2 cup salted butter, 1 teaspoon vanilla extract
Remove the caramel from the heat and add the cornflakes. Stir until the cereal is coated in the caramel, then pour on top of the cooled brownies. Gently spread into an even layer, then cool completely before cutting.
5 cups cornflakes