MAKE THE BROWNIES. Preheat oven to 350 degrees F. Line an 8×8-inch baking dish with parchment paper, letting at least 1 inch overhang on all sides. In a large bowl, combine the brownie mix, butter, milk, egg and vanilla. Mix with a spoon until combined. Pour into the prepared baking dish and bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Let cool completely.
MAKE THE CARAMEL. In a medium saucepan, combine the sugar, heavy cream and corn syrup. Set heat to high and cook, stirring, until mixture reaches 250 degrees (about 10 minutes). Remove from heat and stir in 1 teaspoon sea salt, along with sour cream.
Pour caramel sauce over the cooled brownies. Sprinkle with additional 1/2 teaspoon of sea salt. Refrigerate until caramel is firm, a minimum of 3 hours. Cut into squares and serve.