Add the brownie mix, melted butter, egg, egg yolk, heavy whipping cream, salt, and vanilla to a large mixing bowl. Stir well until everything is evenly combined but be sure not to over-mix your batter.
1 (16- to 18-ounce) box brownie mix, ½ cup salted butter, 1 large egg, 4 Tablespoons heavy cream, ½ teaspoon salt, 1 teaspoon pure vanilla extract, 1 cup chopped chocolate