Chocolate Buttercream

This recipe is from http://www.epicurious.com/recipes/food/views/CUBAN-OPERA-CAKE-108593, which itself looks amazing and someday, we should try out the whole recipe. This time, I changed this recipe quite a bit, but next time, I think I will try to follow it more. I just didn't have all of the ingredients. This one still came out fine, but pretty sugary. If you use shortening in place of butter, I think you have to add some powdered sugar to stiffen it up. The combination makes the frosting easier to work with, and not melt so quickly.

Buttercream
8 ounces imported milk chocolate, chopped (I just used semi-sweet chocolate chips)
1/2 cup sugar
4 large egg yolks (I used 2 eggs)
2 tablespoons water
2 tablespoons light corn syrup
3/4 cup (1 1/2 sticks) unsalted butter, room temperature (I used all vegetable shortening)
(+ I added some powdered sugar to make it firmer, just enough to get the consistency I want)

For buttercream:
Melt milk chocolate in top of double boiler over simmering water, stirring until smooth. Remove from over water. Whisk sugar, egg yolks, 2 tablespoons water, and corn syrup in medium metal bowl to blend. Add 1/4 cup butter. Set bowl over saucepan of simmering water; whisk constantly until mixture reaches 170°F, about 4 minutes. Remove bowl from over water. Using electric mixer, beat until completely cool and thick, about 6 minutes. Gradually beat in 1/2 cup butter, about 1 tablespoon at a time, fully incorporating each addition and stopping occasionally to scrape down sides of bowl. Beat in lukewarm melted chocolate.