Preheat the oven to 350 and grease three 8-inch pans with non-stick spray.
In a large bowl, whisk together the dry ingredients.
Add the wet ingredients and whisk until well combined, then fold in the Oreos.
Divide the batter between the pans and bake for 20-25 minutes, until a toothpick comes out clean.
Set the pans on wire racks, cover loosely with clean kitchen towel and cool completely.
In the bowl of your stand mixer fitted with the whisk attachment, beat the butter for one minute until smooth.
With the mixer on low, gradually add the powdered sugar, meringue powder and salt and mix to combine. Add the vanilla and milk/cream and whip on medium high for 4-5 minutes until light and fluffy.
Fill the cake layers and frost all over with a thin crumb coat of buttercream (I also sprinkled the extra M&Ms between each layer for crunch). Refrigerate for 20 minutes.
Meanwhile, carefully twist apart the Oreos and set aside the frosted side for the eyes.
Roughly chop the unfrosted sides of the Oreos and fold into the remaining buttercream (don’t over-mix, or the crumbs will turn the frosting gray). Frost the cake with the Oreo buttercream.
Press M&Ms to the frosted cookies and stick onto the cake for the monster eyes.
The reason I wait to add the Oreos to the buttercream until after doing the crumb coat is that they would make it very difficult to get a smooth crumb coat on the cake with all the cookies mixed throughout.
Leftover cake should be stored at room temperature in an airtight container for up to 3 days.
Keyword Cake, Cookies and Cream, Halloween, High Altitude