I havent tried it yet but it looks good!
From:
http://www.whimsicalbakehouse.com/bake/bake_detail_matts_fudge.html
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Matt's Fudge
What would life be like without fudge. My son would certainly would not find much pleasure in cake were it not for this creamy and sweet chocolate classic. This recipe comes from my Uncle Matt (the other beloved baker in our family). This fudge has iced his triple Chocolate Cake at the Hudson House in Nyack for as long as I can remember a chocoholics dream come true.
Matts Fudge Icing can be stored in the refrigerator in an air tight container for 2 weeks. To use the chilled icing, bring it to room temperature and beat at medium speed in the bowl of an electric mixer until smooth and creamy. To speed up this process you can lightly reheat the icing in its mixing bowl, over a hot water bath, before beating.
-LH
In a small saucepan over low heat, mix together:
¼ cup granulated sugar
2 large egg yolks
¾ cup heavy cream
1 teaspoon pure vanilla extract
Pinch of salt
Whisk until slightly thickened and coats the back of a wooden spoon (175-180° on a candy thermometer):
Remove from the heat and whisk in until melted:
8 ounces finely chopped semisweet chocolate
3 ounces (3/4 stick) unsalted butter
Whisk in:
¼ cup light corn syrup
2 tablespoons sour cream
Transfer to a bowl, cover with plastic wrap, and chill for 1 hour to overnight.
Yield: approximately 2 ¼ cups
Do you think it could go on thick for a nice filling? i hink i'll just try it and see
Probably. You can use any frosting for a filling. Ganache would probably be a lot easier to make, though.
I really want a fudge icing recipe for icing cakes though so that is why I looked this one up. You know I think I have another recipe somewhere....
This one seems easier to make but I think the first recipe sounds like it would taste better! 
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Chocolate Fudge Icing
Source: Santa's Holiday Treats
Ingredients:
2/3 cup butter or margarine
1 1/3 cup cocoa
6 cups confectioners' sugar
2/3 cup milk
2 teaspoons vanilla extract
Makes: 4 cups of icing.
instructions
Melt butter in saucepan over medium heat. Add cocoa and heat just until mixture begins to boil, stirring constantly until smooth. Pour into mixer bowl. Cool. Alternately add confectioners' sugar and milk, beating to spreading consistency. Blend in vanilla.
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