Heat the oven to 350°F (175°C). Line a 9-by-13-inch baking pan with parchment paper or foil, leaving enough overhang to lift the cooled brownies out easily.

Prepare the brownie batter according to the package directions using the eggs, vegetable oil, and water listed on the box. Stir only until the dry ingredients are incorporated, taking care not to overmix.

Fold about half of the chopped Snickers bars into the batter. Spread the mixture evenly in the prepared pan.

Bake for 25–32 minutes, or until the edges are set and a toothpick inserted near the center comes out with moist crumbs. Do not wait for the toothpick to come out completely clean, since the brownies continue setting as they cool.

Immediately scatter the remaining Snickers pieces over the warm brownies. Drizzle the caramel sauce across the top and sprinkle evenly with the chopped roasted salted peanuts.

Cool the brownies for at least 45 minutes before slicing. For especially clean cuts, refrigerate them for 30–60 minutes first, then add an optional semisweet chocolate drizzle if desired.