Preheat the oven to 350°F. Spray a 9 X 13 inch baking dish with non-stick cooking spray and set aside.: As the oven warms you will notice a dry heat that primes the pan to promote even baking. The right oven temperature ensures the edges brown gently while the center cooks through, creating a uniformly tender crumb. Use an oven thermometer if yours runs hot or cold to avoid an overly dark crust or an underbaked middle. A common mistake is skipping the spray which can make removal messy; coat the dish well so slices lift cleanly.
In a large bowl, mix together the yellow cake mix, pumpkin puree, brown sugar and pumpkin pie spice until combined and smooth but do not over mix. You can use a hand mixer here to make things easier.: The batter will smell warmly spiced as the pumpkin pie spice blooms and the pumpkin gives off a faint, earthy scent. Mixing until smooth distributes the moisture and flavor without developing gluten, which keeps the cake tender. If you overmix, the crumb can become gummy, so stop when streaks disappear and texture is uniform. A typical error is vigorous mixing; use short pulses with a hand mixer to maintain lightness.
Transfer the cake batter to the prepared baking dish and spread evenly. Bake the cake for 20 to 25 minutes. An inserted toothpick should come out clean.: As the batter bakes you will see it rise slightly and the surface take on a pale golden color, releasing a sweet, spiced aroma. The toothpick test is your best cue for doneness; look for golden edges that pull away from the pan and a springy center. If the top is browning too quickly, tent with foil to prevent overcoloring. People sometimes underbake the center, so trust the toothpick rather than just time.
Allow the cake to cool for 15 minutes on a wire rack.: During this rest the cake firms up and the internal steam redistributes, which makes it easier to poke and to hold the sweetened condensed milk. The surface will stop steaming and feel slightly springy to the touch. Removing it too soon from the pan can cause it to crumble, so let it settle on the rack. Avoid rushing to the next step while it is hot, because the holes will not hold properly.
Poke holes all over the top of the cake about 1 inch apart and about halfway through the cake. You don't want to poke the holes through to the bottom of the cake. You can use the end of a wooden spoon or spatula to poke the holes.: The act of poking lets the sweetened condensed milk sink into the crumb, creating pockets of concentrated sweetness. Aim for even spacing so the soak distributes uniformly and each slice benefits. If you poke too deep you may create channels to the pan, which will make the milk pool on the bottom instead of soaking. Be deliberate and use a tool with a rounded end to control depth.
Evenly pour the sweetened condensed milk over the top of the cake making sure it fills all of the holes you just created. Chill the cake in the fridge for 30 minutes to allow the sweetened condensed milk to soak into the cake.: As the milk settles it will sink with a glossy sheen into the tiny tunnels you created, and the surface will look moist but not drowned. Cooling the cake helps the milk stay where it should, soaking gradually into the crumb for a creamy interior. A common blunder is pouring too fast which leaves uneven saturation; take your time and tilt the pan slightly if needed for balance.
Remove the cake from the fridge and carefully spread the whipped topping evenly over the top of the cake.: The whipped topping should be cool and airy, providing a cloudlike contrast to the dense, soaked cake below. Spread gently to keep its volume and create a smooth, even layer that will hold the toffee bits without sinking. If the topping is still too soft, chill briefly before spreading, because a runny layer will mix into the milk soaked top and lose its texture.
Sprinkle the Heath bits over the top. Next, drizzle the caramel sauce over the top of the cake.: The toffee bits add a crisp, crunchy counterpoint and the caramel introduces a sticky, buttery gloss that binds the elements. Sprinkle evenly to avoid clumps of candy in one slice, and drizzle the caramel sauce in ribbons for visual appeal. Beware of overdoing the caramel, which can overwhelm the gentler pumpkin notes; a light hand creates balance.
Finally, place the cake back in the fridge and allow it to chill for a minimum of 3 hours. If you have the time, this cake is truly at its best when it is made the day before and chilled overnight. Serve and enjoy!: As it chills the flavors meld and the textures unify into a sliceable, cohesive dessert with creamy pockets and crunchy accents. Overnight chilling encourages full absorption and a cleaner slice when serving. Cutting too soon will yield loose slices, so be patient and chill for the recommended time to get neat pieces. A final tip is to warm the caramel sauce slightly if it firms, for easy drizzling when serving.