Preheat oven to 350°F. Line an 8×8-inch baking pan with foil and spray the foil with nonstick cooking spray; set pan aside.
Cut each Fun Size Twix bar into thirds and set the pieces aside. Unwrap the 10 caramel candies and set them aside for the sauce.
Place the chopped unsweetened chocolate and cubed butter in a microwave-safe bowl. Microwave at 50% power in 30-second increments, stirring between each increment, until the butter is melted and the chocolate is smooth.
Stir the chocolate-butter mixture until fully combined and smooth. Whisk in the sugar until incorporated, then whisk in the eggs, vanilla extract, and salt until the mixture is smooth and uniform.
Gradually add the all-purpose flour to the chocolate mixture, stirring just until no dry streaks remain (do not overmix).
Spread half of the brownie batter evenly into the prepared pan (use a spatula to smooth the surface). Arrange the Twix pieces in an even layer over the batter.
Spoon the remaining half of the batter over the Twix layer and spread gently to cover the candy pieces as evenly as possible.
Bake at 350°F for 30–35 minutes, or until a toothpick inserted near the center comes out with a few moist crumbs (avoid overbaking). Remove the pan from the oven and place it on a cooling rack to cool completely.
Once the brownies are completely cool, lift them from the pan using the foil and transfer to a cutting surface. Cut the brownies into squares.
To make the caramel sauce, place the unwrapped caramel candies and heavy cream in a microwave-safe bowl. Microwave at 50% power in 30-second increments, stirring frequently, until the caramels are melted and the mixture is smooth. Drizzle the warm caramel over the cut brownies and serve immediately.