Start with the Fiesta Fantasy recipe from Hershey's Kitchens. The only modifications I made to the cake itself is that I leave out the coffee and I only make three layers instead of four. I also disregard the chocolate frosting recipe, because I think a simple chocolate ganache is all the topping this cake needs. Well, that and shamrock sprinkles for St. Patrick's Day. Obviously.
My major modifications come during the filling part. Simply take 1.5 cups of heavy whipping cream and beat it until stiff. Add one tablespoon of mint extract and a bit of green food coloring. Beat it some more and then let it sit in the fridge for a few hours. Or even overnight. The filling is flexible like that.
When the cake layers have cooled completely, start layering the cake as follows: cake, mint filling, cake, mint filling, cake, chocolate ganache, sprinkles. There! Done! Wasn't that easy? To make sure the cake holds up, be sure to store it in the fridge until you're ready to serve it. If you'll be travelling with the cake, it can go in the freezer for a few hours to really firm it up before making the trip.
This is the perfect combination of mint and chocolate in cake form. Give it a try for St. Patrick's Day!