Chocolate buttercream frosting

We celebrated my husband's birthday a few times this year, but our first celebration was when my family was in town last week.  He asked for a marble cake with chocolate buttercream frosting, so I went searching for a frosting recipe. (I went box route on the cake - although I'm sure a marble cake homemade would not be too hard!)  I found this recipe, and it turned out wonderful!  Here it is:

Ingredients
1/2 cup solid vegetable shortening
1/2 cup (1 stick) butter or margarine, softened -I used butter
3/4 cup cocoa or three 1 oz. unsweetened chocolate squares, melted - I used cocoa
1 teaspoon clear vanilla extract
4 cups sifted confectioners' sugar (approximately 1 lb.)
3-4 tablespoons milk
Makes: About 3 cups of icing.

Stiff Consistency: In large bowl, cream shortening and butter with electric mixer. Add cocoa and vanilla. Gradually add sugar, one cup at a time, beating well on medium speed. Scrape sides and bottom of bowl often. When all sugar has been mixed in, icing will appear dry. Add milk and beat at medium speed until light and fluffy. Keep bowl covered with a damp cloth until ready to use. For best results, keep icing bowl in refrigerator when not in use. Refrigerated in an airtight container, this icing can be stored 2 weeks. Rewhip before using.

For Thin (Spreading) Consistency Icing: Add 3-4 tablespoons light corn syrup, water or milk.

Since we were using it to frost a cake, I added 3 tbsp of water and it was very spreadable.  I wasn't sure how much 3 cups of icing was (the yield) but it was plenty!  I used a very small amount between the layers and I had leftovers.  So, if you use for a cake, feel free to load up the bottom cake layer - I promise you will have enough!  Next, I'm going to try the stiffer consistency on my favorite Magnolia bakery cupcakes. Yum!