Preheat oven to 350 degrees. Line a 9 x 13 pan with foil and spray with cooking spray.: When you preheat, you will notice the warm oven air and the faint metallic hum of the heating elements. Getting the oven up to a steady 350 degrees ensures the brownie batter begins baking immediately and sets the layers rather than slowly collapsing. A common mistake is skipping full preheat, which can cause uneven rise and longer bake times.
Make brownie mix according to package directions. Pour half of the batter into the bottom of the pan and spread to coat. Line the brownie mix with the Oreos and then top them with the remaining brownie mix.: The foil gives a clean lift out when the bars are done, and the spray prevents sticking, so you can maintain neat edges. Press the foil into corners until it lies flat, and be attentive to seams that might trap batter. If you skip the foil, cutting and transferring squares can be messy and fragile.
In a large bowl, beat cream cheese and sour cream until combined and no lumps remain (your ingredients need to be at least room temperature for this!). Beat in egg and sugar. Pour over brownie mix and spread to the edges of the pan. Sprinkle Andes chips on top of the cheesecake layer.: Mixing the batter creates a glossy, thick texture that will become your fudgy base. Stir just until combined to avoid developing excess gluten, which leads to cakier results. Overmixing is a frequent error that makes brownies dryer and less tender.
Bake for 35-40 minutes. Cool completely before cutting. Store in refrigerator.: The first layer should be evenly spread so the Oreos sit on a stable surface. Use a spatula to smooth the batter to the corners, watching for air pockets. If the batter is too runny it will not hold the cookies in place, so ensure you followed package ratios precisely.
Line the brownie mix with the Oreos and then top them with the remaining brownie mix: Place the Candy Cane Oreos in a single even layer, slightly pressing them into the batter so they adhere but maintain definition. Pour the remaining batter over the cookies and try to cover them without smearing the edges. A common slip up is crowding the cookies; keep them evenly spaced for consistent slices.
In a large bowl, beat cream cheese and sour cream until combined and no lumps remain: The mixture should transform into a smooth, silky mass without streaks. Use room temperature cream cheese and sour cream so they blend seamlessly, and beat gently to avoid aerating the filling which can create cracks while baking. If you rush with cold dairy, you will see lumps and an uneven bake.
Beat in egg and sugar: Adding the egg and sugar incorporates structure and sweetness into the cheesecake layer. Beat until glossy, scraping the bowl sides so everything is uniform. Overbeating can create air bubbles that lead to pocking on the surface after baking, so stop once smooth.
Pour over brownie mix and spread to the edges of the pan: The filling should glide across the brownie surface, forming an even blanket over the cookies. Use an offset spatula for gentle, controlled spreading to avoid disturbing the layers below. If the filling is uneven, it can cause thin spots that bake faster and dry out.
Sprinkle Andes chips on top of the cheesecake layer: Scatter the Andes Baking Chips evenly so they melt into little mint pockets. You will see them begin to soften as the pan rests before baking, which is a good sign. Avoid piling them in one spot; uneven distribution leads to clusters that weigh down the filling while baking.
Bake for 35 to 40 minutes: As the pan bakes you should notice a gentle aroma of chocolate and peppermint, and the cheesecake top will set but might still have a slight jiggle in the center. The edges will start to pull away from the foil slightly when done. Baking too long will dry the layers, so begin checking at 35 minutes. A common mistake is assuming the bars need heavy browning; gentle set is the cue.
Cool completely before cutting: Cooling allows the cheesecake layer to firm and the flavors to meld, and you will feel a dense, cool texture when you touch the foil. If you cut while warm, the bars will be gooey and fall apart. Patience here yields clean, beautiful squares.
Store in refrigerator: Chilled bars keep their structure and the peppermint stays crisp, so store them in an airtight container in the refrigerator . They will taste firmer and the layers more distinct after a few hours. Leaving them at room temperature for too long can soften the cheesecake into a looser slice that is harder to serve.