Preheat oven to 350 degrees F. Line a 15"x 10"x 1" baking sheet with tin foil, so the foil comes up and over the edges, and lightly coat with non-stick spray. Set aside.: When you preheat, notice the way the kitchen gradually warms and the oven light glows, that even heat is crucial for an even rise and uniform bake. The foil that wraps up over the edges helps lift the finished slab from the pan easily, and the non stick spray ensures clean release. A common mistake is not lining the pan properly, which can make removal difficult and lead to broken bars. If your oven runs hot, consider an oven thermometer to confirm the temperature, as a hotter oven will brown the edges too fast while leaving the center underbaked.
Whisk together the flour, sugar, baking powder, pumpkin pie spice, baking soda and salt in a large bowl.: As you combine these dry ingredients you will notice the aroma of the pumpkin pie spice bloom, signaling the warm notes that will carry through the bake. Whisking evenly distributes leavening so there are no bitter pockets of baking soda or clumps of spice, and that uniformity creates a consistent crumb. A common error is hurried measuring, which can affect texture; spoon and level your flour to avoid packing it in and making the bars heavy.
In a separate bowl whisk together the 4 eggs, can of pumpkin puree and vegetable oil. Pour the wet ingredients into the bowl of dry ingredients and whisk to combine. Pour batter into prepared baking sheet and spread into an even layer.: The wet mixture should look glossy, with eggs fully emulsified into the oil and pumpkin puree . When you fold wet into dry, mix until just combined; over mixing develops gluten and makes the bars chewy. Spread the batter with an offset spatula to an even thickness so baking is uniform. If the batter is left lumpy or uneven, you may end up with thin spots that bake faster, a common pitfall to watch for.
Melt 1/2 cup of the mini chocolate chips in the microwave on 30 second bursts, until smooth. Beat the melted chocolate, cream cheese, sugar, egg and milk together until smooth.: The melted chocolate chips and softened cream cheese will create a glossy, lush mixture. Heat in short bursts and stir between bursts to prevent scorching; chocolate can seize if overheated. When you beat these ingredients, aim for a silky, lump free texture so dollops will ripple nicely into the pumpkin batter. A common mistake is overheating the chocolate, which creates a grainy texture, so stop once it is pourable and stir thoroughly.
Drop dollops of the chocolate mixture all over the pumpkin cake and swirl into the cake with a knife. Sprinkle the remaining 1/2 cup mini chocolate chips all over the cake.: As you drop the dollops, listen for the slight thunk of the spoon meeting the batter, and watch how the cream cheese mixture sits atop the pumpkin base. Use a knife to create broad swirls that pull the two components together visually, making pretty marbling. The remaining mini chocolate chips provide textural contrast and pockets of melty chocolate after baking. Avoid swirling too aggressively, which can overmix and blur the marbling; gentle ribbons yield the most attractive pattern.
Bake for 25 to 30 minutes. Let cool completely and cut into squares. Store in fridge for up to 3 days. Enjoy!: While baking the kitchen will fill with a warm spice scent and the top should set, losing its wet sheen. I watch for the edges to pull slightly from the foil and for the center to feel set when lightly touched, which indicates doneness. Let cooling be patient, as cutting too soon causes the bars to crumble; chilling in the fridge firms them for neat slices. A common mistake is underbaking by a few minutes, leaving a gummy center, so if unsure give it a few extra minutes and check for the described visual cues.
