If you are looking for a simple cake that looks impressive - this is the one to make! I was not planning on posting so I don't have exact directions, but feel free to ask questions. Next time I make it I'll measure out more exactly and update the post.
Devil's Food Cake Mix (and necessary ingredients from back of box)
1 Can Chocolate Icing (I use the Duncan Hines Chocolate Fudge)
Miniature Reese's Peanut Butter Cups (not the tiny ones that are unwrapped)
1 Decorating Tip/Coupler and Ziploc bag (you'll see!)
Put your Reese's Miniatures in the freezer!
Make the cake according to package for two round cakes, when done take out of pans and put in freezer. (I stack mine with paper plate or waxed paper between them (this helps to flatten the bottom layer somewhat)
Visit Cinnamon and Spice Site and use her peanut butter filling recipe. One note - after mixing the XXX Sugar and the Peanut Butter/Vanilla it will be crumbly until you add the milk.
Put the icing in the refrigerator to firm it up slightly.
Take the cakes out of the freezer, put one on your cake plate (even out top if too rounded - I actually put mine upside down rather than cutting it (see note above for freezing) because the roundedness was not too bad - this gives the bottom a very smooth look after cutting.)
While it's all setting up, take the Reese's Cups out of the freezer, unwrap and chop into small pieces (it's judgement but I had most my cups cut up into about 8 littler pieces.) (I chopped enough to fill two small cereal bowls)
Spread the peanut butter filling across the one layer (reserving some for top decorations - I had plenty to spare so use as much as you'd like in the middle).
Take a Quart Sized Ziploc Freezer Bag and cut a small hole in the corner, insert your coupler and screw on your tip. Add the remaining peanut butter frosting to the bag (I use a cake spreader to help get it down near the bottom without needing to squeeze as much). Put in refrigerator to set back up.
Sprinkle a loose layer of the chopped Reese's over this layer (about 1/4 of what you've chopped). This should not cover like in the picture - if you do the cake doesn't stick together well and it becomes a bit overwhelmingly rich.
Top with second cake layer. Frost cake with one layer of chocolate icing (save some for smooth coat). Put back in the freezer for about 10 minutes to help set the initial coat. Take out of freezer and add a smooth coat of the remaining chocolate icing on top of existing coat - making sure to put fresh icing on top. Sprinkle remaining chopped Reese's on top of cake, filling in but leaving a small border around the edge. The Reese's should cover the top of the cake completely though, no open spaces. (I used a little more than 1/2 of one bag of the miniatures for this)
Decorate the outside of the cake and the bottom round using the Peanut Butter frosting you put aside in the Ziploc Bag earlier. (Looking back I would have done stars on the bottom if I could re-do, the shell wave did not hold up. If you do want to do something, keep putting the frosting back in the refrigerator to keep it firm.)
That's it - Keep cake in the refrigerator until ready to serve - the peanut butter filling holds up better and I think tastes better cold - but your call on whether to get to room temp prior to serving !