Preheat the oven to 350 degrees F and grease a 9x13-inch cake pan, set aside.
Mix together all the cake ingredients until fully combined and transfer to the prepared cake pan. The batter will be thick and you will need to use a rubber spatula to spread it out.
Bake for 28-32 minutes or until a cake tester inserted into the center comes out clean.
Let cool for 30 minutes and poke holes all over to top with the handle of a wooden spoon.
Filling
Whisk together the pudding mix and milk for 2 minutes, then allow to thicken slightly.
Spread the pudding over the top of the cake all the way to the edges.
Frosting
In a large bowl or stand mixer fitted with a whisk attachment, whisk together the frosting ingredients until thick and fluffy.
Spread the frosting over the top of the cake all the way to the edges.
Topping
Add the Oreos to a food processor and pulse until fine crumbs form. (Or place in a plastic Ziploc bag and crush with a rolling pin.) Sprinkle the Oreo crumbs over the top of the cake.
Write R.I.P. on the Milano cookies to make tombstones and then press them into the frosting so they stand up.
Press the ghost Peeps into the frosting so they stand up. If they have a hard time doing so, you can use toothpicks to help secure them.
Add the candy pumpkins near the ghosts and tombstones. Add sprinkles, if desired. Cut into 12 slices before serving.
Notes
This cake should be refrigerated within 2 hours after frosting or the frosting can get really dark and dense. It will keep covered in the fridge for up to 4 days.
The cake can be baked up to one day in advance and then frosted and decorated just before serving.