how to make a dark chocolate frosting that is hard when it sets? almost like magic shell?

Yes, ganache. It's easy & delicious - especially when you use *good* chocolates. Here's a recipe:

http://www.joyofbaking.com/ganache.html

Make your frosting and put coconut oil in it. Coconut oil stays liquid above 75°F, or so. You can put it on the dessert while it is warm and it will spread easily...then cool your dessert below that temp and it will be hard.

If you have a recipe which you are wanting to use for the fosting...go ahead and use it and just sub coconut oil for the shortening. If you want to make it really thin like magic shell, here is a recipe for making your own magic shell:

Homemade Magic Shell Topping

1 cup coconut oil

about 1-2 cups semi-sweet, bittersweet, or dark chocolate chips or chunks

Melt the coconut oil over very low heat in a small saucepan.

A little at a time, whisk in the chocolate chips, until the mixture resembles very thin, chocolate syrup.

That's it! This stuff stores at room temperature. IN a cold room, make sure to heat over very low heat on the stove.

Like on a Hostess cupcake? Look for a recipe for chocolate ganache.

Use dark choc chips and make a ganache.