Make the brownies: Preheat the oven to 325˚. Butter a 10-inch cast-iron skillet. Combine the butter and chocolate in a medium saucepan over medium heat. Cook, stirring, until melted and smooth, about 3 minutes. Remove from the heat and stir in both sugars until dissolved. Whisk in the eggs, one at a time, until smooth and glossy, then whisk in the vanilla. Whisk in the flour and kosher salt until combined, then whisk in the pecans. Spread the batter evenly in the prepared skillet.
Transfer the skillet to the oven and bake until the brownies are slightly cracked on top and a toothpick inserted into the center comes out mostly clean, 35 to 40 minutes. Transfer to a rack and let the brownies cool in the skillet.
As soon as the brownies come out of the oven, make the toffee topping: Combine the brown sugar, butter and corn syrup in a medium saucepan over medium-high heat and bring to a boil, stirring occasionally. Reduce the heat to medium to maintain a steady simmer and cook, without stirring, until the sugar is completely dissolved, about 2 minutes. Carefully pour in the heavy cream (it might boil up slightly) and simmer until thickened, 5 to 8 minutes. Remove from the heat and stir in the pecans. Let cool until thickened but still pourable, about 20 minutes.
Pour the topping on the brownies and let set, 1 hour. Sprinkle with flaky sea salt, if using. Serve with ice cream.