Preheat oven to 350 F.: Warm air and the subtle scent of dry heat tell you the oven is ready, and preheating ensures even rise and set of the batter. You should hear the faint click as the oven cycles and see the temperature stabilize. If you skip preheating, the brownies may bake unevenly, with edges overcooking before the center sets, so always wait until the oven reaches temperature.
Spray a 9 x 13 in baking dish with cooking spray and line with parchment paper. Set aside.: The slick surface of the sprayed pan helps the parchment stay in place while the parchment provides a lift out for easy removal. When you press the paper into the corners, it should sit flat without wrinkles. A common error is not trimming excess paper, which can fold into the batter; keep the lining neat for clean edges when you lift the bars out.
Combine caramels and 1/3 cup evaporated milk in a microwave safe container. Heat on high heat in 30 second intervals, stirring in between, until fully melted. Set aside.: The caramel will go from stiff squares to a molten, glossy sauce, releasing a warm, buttery aroma that fills the kitchen. Stirring reduces hot spots and prevents scorching, which can make the sugar grainy and separate. If you overheat, the caramel may become thick and difficult to drizzle, so heat gradually and stop when the mixture is silky smooth.
Combine cake mix, melted butter, 1/3 cup evaporated milk, mini chocolate chips, and chopped nuts in a bowl. Stir just until combined.: As you fold these elements, you will notice pockets of melted butter and streaks of chocolate chips; gentle mixing yields a dense, cohesive batter while preserving texture. Stirring too vigorously encourages more air, creating a cakier result, so stop when the ingredients are incorporated. Be mindful of temperature, if your butter is too hot it can soften the chips prematurely.
Spread half of the cake mixture onto the bottom of the baking dish and bake for 7 minutes.: Press the batter into an even layer, then watch the edges as they begin to darken and pull away slightly from the pan during the brief bake. This partial bake sets a sturdy base so the caramel and chips do not sink too deep. If you underbake here, the bottom layer may not support the toppings and could become soggy, so trust the full seven minutes.
Remove from the oven and sprinkle on one cup of chocolate chips. Drizzle the caramel sauce on top. (If caramel has cooled to the point that it cannot be drizzled, return to the microwave for 10 seconds and stir.): The contrast of warm chips and molten caramel creates a glossy, inviting layer; you will hear faint plinking as some chips settle. If the caramel has cooled, a quick heat loosens it back to a ribbon like pour. Avoid overheating the caramel at this stage, which can make it very runny and soak too far into the batter.
Spread the remaining half of the cake mixture over the top of the caramel and chocolate chips. It's okay if you don't get 100% coverage. Bake for 15-18 minutes.: When you press the top layer lightly with a spatula, it should form a patchy blanket that will bake into a thin crust with a soft interior. As it bakes, expect a rich chocolate aroma and small steam puffing from the surface. Overbaking will dry the bars, so start checking at 15 minutes; a toothpick with a few moist crumbs indicates perfect doneness.
Serve immediately with ice cream or let cool before cutting into bars.: If you serve warm, the caramel and chocolate chips will be molten, creating an oozing, luxurious bite that pairs beautifully with cold creaminess. Cooling gives cleaner slices and a firmer texture. Cutting too soon can lead to messy squares, so use a sharp knife and gentle rocking motion after cooling for neater presentation.
Store in an airtight container for up to 4 days.: Proper storage locks in moisture and prevents the bars from picking up off flavors from the fridge. The chilled texture will firm the caramel , making the bars chewier. Avoid stacking warm bars in a sealed container, which can trap steam and lead to sogginess; allow them to cool to room temperature first.