Ingredients

8 ounces semisweet chocolate, coarsely chopped
1 cup heavy cream
1 tablespoon unsalted butter, at room temperature

 

Instructions

Place chocolate in a medium heatproof bowl; set aside.
Place cream in a small saucepan over medium heat and bring to a simmer.
Pour over chocolate and let stand until chocolate has softened, (about 5 minutes).
Add butter and stir until smooth.
Let cool slightly before using.

Game plan:
When working with ganache, make sure it’s warm enough to pour.
If it’s too stiff, fill a small saucepan with an inch of water and bring it to a simmer over low heat.
Set the bowl of ganache over it and melt, stirring, to the desired consistency.

I've found that the best bet (so that you don't have a mess on your cake platter) is to place cake onto a *cardboard cake circle.
Place circle on a wire rack, and pour ganache over top of cake. 
What ever drips over, will fall through rack for easy clean up when done.
Place cake, cardboard and all, onto your cake platter!

Wilton® makes this cake circles, (they also come in squares and rectangles) and are sold in most craft stores.