I used this Pound Cake Recipe (doubled with 1/2 hour less cook time, I almost burned the dickens out of the whole thing) and this Marshmallow Fondant Recipe. I also used strawberry jam for the filling. And I set the top piece of a birthday candle in the fondant '2'.
After baking, leveling and assembly I froze the cake for 2 hours before covering. Unfortunately I ran into a mishap with the fondant. I found out the hard way that all marshmallow is not created equal (I had purchased a store brand that I never used before) and I had the most fragile fondant ever that fell apart and tore at the slightest provocation. After much struggle I messily covered the cake (this bad fondant left it much lumpier than I wanted) and hoped that it wouldn't distract from the finished piece.
I overlayed all my small fondant work over my rough printed template before I transferred it to the cake. Then I brushed the backs of the pieces with a little water to help it bind to the fondant covering. Finally, to cover up the very rough edges, I whipped together some quick frosting. And Ta-Dah! Portal 2 Cake! The cake was saved for the next day when it was devoured by our D&D group.
If I had to do it again there are several things that I would execute differently, but mishaps and all I don't think I could hope for a better out come :)
Here is a little tidbit of video from my husband's birthday.