Step 1: Preheat oven to 350°F (175°C). Grease a 9x13-inch baking pan and line the bottom with parchment paper for easy removal.
Step 2: Prepare the chocolate cake mix according to package directions, combining cake mix, eggs, oil, and water in a large bowl. Mix until just combined.
Step 3: Pour batter into the prepared pan. Bake for 22-25 minutes, or until a toothpick inserted into the center comes out with moist crumbs. Let cool in the pan for 10 minutes.
Step 4: While warm, use the handle of a wooden spoon or a chopstick to poke 30-40 holes evenly across the top of the cake, pressing about two-thirds down.
Step 5: Evenly pour the entire can of sweetened condensed milk over the warm, poked cake, ensuring it seeps into all holes.
Step 6: Drizzle ½ cup of the caramel sauce in a zigzag pattern over the cake. Refrigerate uncovered for 10 minutes.
Step 7: Scatter most of the chopped Snickers bars evenly over the cake, gently pressing some pieces into the holes. Reserve a small handful for garnish.
Step 8: Cover the pan tightly with plastic wrap and refrigerate for at least 1 hour (or up to 2 hours) to chill completely.
Step 9: For the whipped cream, chill a large mixing bowl and whisk attachment. Beat heavy whipping cream, powdered sugar, and optional vanilla extract on medium-high speed until stiff peaks form.
Step 10: Once the cake is chilled, spread the whipped cream evenly over the top.
Step 11: Warm the remaining ¼ cup caramel sauce and drizzle over the whipped cream. Drizzle with 2 tablespoons of chocolate sauce.
Step 12: Sprinkle the reserved chopped Snickers and the chopped roasted salted peanuts over the top.
Step 13: Allow the cake to rest in the refrigerator for another 30 minutes before serving. Slice and enjoy!