Preheat an oven to 350℉ (180℃). Grease 3 8-inch cake pans with baker's floured spray and/or line pans with parchment rounds.

Mix together dry ingredients (flour, sugar, cocoa powder, baking soda, and salt) in the bowl of a stand mixer with a paddle (or hand mixer or whisk) until well combined.

Add in bits of softened butter, mixing until no lumps of butter are visible and the mixture looks crumbly.

Whisk together eggs, buttermilk, and vanilla. Add to dry ingredients and mix until no dry ingredients are visible. Scrape down the bowl and beat for another 20 seconds.

Fill the three pans evenly - I find a kitchen scale helpful for this part, and each of mine weighed out to about 550g. Bake for 30-35 min or until a toothpick inserted comes out clean. Allow cake layers to cool for 10-15 minutes on a wire cooling rack before removing from pans, and cool completely before frosting. Set in the fridge or freezer to accelerate the cooling process if desired.

Beat together softened cream cheese and butter; slowly add in powdered sugar alternating with milk until frosting reaches desired consistency. Add vanilla, salt, and cookie crumbs and beat until well combined.

Place heavy cream and white chocolate chips in a microwave-safe bowl, and microwave on 50% power in 30-second intervals until smooth, stirring in between.

Once your cakes are cool, level them (if needed/desired). This can be done with a cake leveler or a large serrated knife and a ruler. Place a smear of frosting on your cake circle (to keep the cake from sliding while you decorate it) and center your first cake layer in the center of the circle. Spread the layer with frosting. Repeat the process with your remaining cake layers.

Now you're ready to crumb-coat . If you're unfamiliar with crumb-coating, it's just what it sounds like - a thin layer of frosting over the entire outside of the cake to keep crumbs out of your final layer.

Once your crumb coat has set (this takes about 5-10 minutes in the fridge), add your final layer of frosting and smooth. I like to use an offset spatula and bench scraper for this part. Once your cake is covered, place it into the freezer to set the frosting, and to chill the cake so that the mirror glaze sets when you pour it on.

Okay - the fun part! Once your drip mixture has cooled - it should be slightly warm and still liquid - remove your cake from the freezer and set the glass on its side on top of the cake. Pour or pipe the mirror glaze into the cup and onto the top of the cake, creating a 'milk spill' design on the top.

Place your Oreos/Oreo pieces around the cup on top of the cake - I used small amounts of frosting to hold them in place. If desired, pipe some frosting and add Oreo cookies/cookie pieces around the bottom edge of the cake as well.

And you're done! Cut in and enjoy!