I adapted this from the birthday cake Matt made me. You might have to play around with it a little to make it what you like.
1 cup heavy whipping cream (very cold)
1/4 cup powdered sugar
2 tablespoons cocoa powder
1/2 teaspoon vanilla extract
First, put the bowl in the freezer for about 10-15 minutes (I also put the cream in there). Whip the cream with a hand or stand mixer until peaks form. I aimed for somewhere between soft and stiff because you mix it a little bit after. Add the sugar, cocoa, and vanilla and mix until blended. Scrape the sides of the bowl, and you are ready to go.
I used this to frost 2 dozen cupcakes. But we prefer to have a lighter coating of frosting. You might want to double the recipe if you like extra frosting. I also lightly dusted them with cocoa powder for decoration (on a side note: do not put the cocoa powder in the sieve while it is above the cupcakes or it will be a heavy coating not a light dusting).
Matt's recipe called for cream cheese and instant coffee (it was a tiramisu cake).