Preheat the oven to 325°F.
Prepare the brownie batter according to the package instructions.
Transfer the brownie batter to a 9-inch square baking pan lined with parchment paper.
Bake until a toothpick inserted into the center comes out clean, about 30 minutes.
While the brownies are baking, melt butter with brown sugar, corn syrup, heavy cream, vanilla, and a pinch of salt in a saucepan.
Simmer for a few minutes, then take off the heat and stir in the pecans.
Increase the oven temperature to 350°F.
Spread the pecan mixture over the brownies and bake again until the topping is bubbling, about another 30 minutes.
Let the brownies cool to room temperature before cutting.
Sprinkle with flaky salt if desired.
