Preheat an oven to 350 degrees F (180C). Grease 3 8-inch cake pans with baker's floured spray (or line pans with parchment paper).
In the bowl of a stand mixer with the paddle attachment, combine the flour, sugar, cocoa powder, baking soda, and salt. Add in bits of softened butter until it’s no longer visible in the dry ingredients 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 leveling and frosting. Set in the fridge or freezer to accelerate the cooling process if desired.
If you want to fill the center of your cake with M&Ms and chocolate chips, use a glass or medium-sized biscuit cutter to remove center from two of the three cake layers. (It works best to do this once your cake layers have cooled; they’re much more likely to crumble if you cut them when they’re warm.)
Chocolate Cream Cheese Buttercream frosting
Beat together softened cream cheese and butter; slowly add in powdered sugar and cocoa powder alternating with milk until frosting reaches desired consistency. Add vanilla and salt and beat until well combined.
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 - if you've cut centers out of two of your cake layers, those will be your bottom two layers. Fill the empty centers of your cakes with M&Ms and chocolate chips. Add a layer of frosting over the second cake layer, and put the final layer on top.
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 fridge or freezer to set the frosting.
Chocolate ganache drip
Meanwhile, make your ganache. Place ingredients in a microwave-safe bowl and microwave in 30-second intervals on 50% power, stirring in between until smooth.
Slowly drizzle the ganache around the upper edge of the cake, pausing every inch or so to let drips of ganache run down the side of the cake. Return cake to the fridge to set the drips.
Decorating
To add the finishing touches to your cake, put the remainder of your chocolate frosting into a piping bag, and pipe swirls of frosting around the upper rim of the cake – I alternated large with small swirls for a little more texture/variety. Place an M&M on top of each frosting swirl.
Now the fun part – the M&M box! I cut one of the small tabs from the side of the box I wanted angled closest to the cake, so that I could easily slip a wood skewer into the box. Press the skewer down into the cake until your box is at the desired height- I left mine about an inch off the top of the cake. (If you skewer shows at all, you can either cut it a little shorter or cover it with some frosting).
Add M&Ms to the top of the cake – I went a little crazy at this part since I had frosting swirls around the rim to hold everything in place! Add small bits of frosting and more M&Ms and chocolate chips around the base of the cake if you’d like, or you can leave them off for a more simple finish.
And that's it - you're done! Find someone to give you a high-five ;) and enjoy!
Video
Notes
Please note nutrition information is an estimate and may not be exactly accurate.