Preheat oven to 350 degrees. Line a 9 x 13 pan with foil and spray with cooking spray.: As you preheat, you will notice the oven warming and the faint scent of dry heat, which primes the pan for even baking. Preheating ensures that when the batter hits the pan it begins to set immediately at the edges, producing that classic brownie crust. A common error is putting the pan in a cold oven, which results in uneven rising and a gummy center. Make sure your oven reaches a steady 350 degrees before inserting the pan.
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.: Lining the pan with foil makes removal and slicing simple and neat, and a light coating of cooking spray prevents sticking while allowing the brownies to crisp at the edges. The foil also catches any overflow and helps the bars cool evenly. Skipping the spray often leads to torn edges when trying to lift the brownies out, so be generous with the non stick spray.
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.: Follow the package instructions for combining the boxed mix with the specified eggs , water, and oil , whisking just until smooth to keep the batter dense and fudgy. You will see a glossy, thick batter that drips slowly from the spoon, a good sign of proper mixing. Over whisking introduces air and can make them cakier, so blend until uniform and stop.
Bake for 35-40 minutes. Cool completely before cutting. Store in refrigerator.: When you spread the first half of the batter, it should form an even, slightly sticky layer that settles into the corners. Use a spatula dipped in warm water to get a smooth surface without tearing the foil. If the batter is too thick to spread, gently warm the spatula to ease movement. Pressing too hard can cause the batter to thin out unevenly.
Line the brownie mix with the Oreos: Place the Oreos in a single layer over the base batter, spacing them so they cover the surface; you will hear little clicks as each cookie meets the batter. The cookies will give distinct chocolate and peppermint pockets; aim for gentle placement so they do not sink completely. Pushing them too deep will reduce the visible cookie layer in the finished bars.
Top them with the remaining brownie mix: Spoon the remaining batter over the Oreos in dollops and gently spread to cover, aiming to seal the cookies between brownie layers. You want a top layer that is even but not overworked, which helps create contrasting textures. If you drag aggressively, you risk dislodging the cookies and creating uneven thickness.
In a large bowl, beat cream cheese and sour cream until combined and no lumps remain: As you beat the cream cheese and sour cream , you will see the mixture loosen into a glossy, smooth texture, and the smell becomes slightly tangy and fresh. Be sure both ingredients are at room temperature for the smoothest result. Working with cold dairy leads to lumps that are hard to incorporate and can cause an uneven cheesecake layer.
Beat in egg and sugar: Add the egg and sugar and continue beating until fully integrated, resulting in a lighter, homogenous filling that will spread evenly over the brownie layer. The sugar helps the filling set while the egg gives structure, so make sure they are fully combined. Adding the egg cold or under beating may create streaks or pockets of uncooked egg in the final bake.
Pour over brownie mix and spread to the edges of the pan: Pour the cheesecake filling across the top and use a spatula to coax it to the edges, creating a sealed, even blanket. You should see smooth, creamy waves that settle without large air pockets, which sets into a glossy sheet when baked. Filling that is too thin will not set properly, while over filling can lead to spillover during baking.
Sprinkle Andes chips on top of the cheesecake layer: Scatter the Andes Baking Chips evenly so the warm cheesecake gets little minty melted pockets, giving bright flecks across the surface. As they warm, the chips lose sharp edges and sink slightly, creating tiny glossy dots of mint. Piling them too thickly in one area causes uneven melting and can make parts of the top overly sugary.
Bake for 35 to 40 minutes: During baking you will notice the edges pull slightly from the foil and the center will look set but slightly jiggly, which indicates the perfect doneness for a creamy cheesecake top that is not overdone. The aroma will deepen with chocolate and a faint peppermint lift as the Andes Baking Chips soften. Overbaking will dry the cheesecake and make the brownies crumbly, so start checking around 35 minutes and tent with foil if the top browns too quickly.
Cool completely before cutting: Cooling lets the layers firm up and the cheesecake settle so your slices are clean and hold their shape, and refrigeration enhances the texture contrast between the dense brownie and the silky cheesecake. You will notice the sheen on the top dull as it cools, a good cue that it is ready to be sliced. Cutting too soon will yield squashed pieces and messy edges, so be patient and wait until fully cooled.
Store in refrigerator: Once cut, refrigerate the bars to keep the cheesecake layer stable and to preserve freshness, where they will keep well for several days and even taste better as flavors meld. Chilled bars slice neater and have a firmer mousse like bite. Leaving them at room temperature too long can make the cheesecake soft and reduce shelf life.
