This will require 6 ounces of a good Euro-style bittersweet chocolate (like Callebaut or Ghiradelli) and one ounce of clarified butter.
Tempering chocolate isn’t terribly difficult, but it does require precise control of temperature. You’ll need a digital thermometer, they’re less than $20 at most kitchen supply stores. If you don’t want to go to the trouble of a tempered finish, you can just spread melted chocolate over the top of your cake. It’ll firm up just fine, though it won’t have the sheen or the snap of tempered chocolate.
Put the chocolate into a glass or crockery bowl. Melt in the microwave in the same way you’d melt chocolate for a ganache, zapping it on high for bursts of 10 seconds or so. Meanwhile, prepare two bowls of water that the chocolate bowl will fit into comfortably. Fill one with ice water, and one with hot water.
When the chocolate mixture is finished melting it will likely be over 100 degrees Fahrenheit. Leaving the thermometer in the chocolate, dip the bowl of chocolate into the bowl of ice water and stir until the temperature comes down to between 80 and 84 degrees (it will start to firm). Immediately put the chocolate into the hot water bowl and bring it back up to right about 89 degrees.
Remove the chocolate bowl from the hot water and immediately stir in the clarified butter. Promptly spread a thin layer of chocolate over your cake with an icing spatula and allow it to set.