The exploding-cake effect at the end was done by Doug Bayne, the guy who does the graphics, live in the studio. He simply smashed the cake with a green plastic cricket bat, then removed it digitally later.
The version with 3 explosions (and hence 3 whacks) was the version aired, but on the first attempt with a different cake Doug managed to smash it completely (although mostly on one half) with one hit. However, they decided to go with the 3-whack version because they felt the timing was better.
In reaction to the repeated skit in the previous week's episode about Chris's reaction to the I Hate Chris advertisements (for the TV show Everyone Hates Chris[/u)], at the beginning of this weeks episode when the stars warm up the audience, 3 members of the audience held up We Hate Chris signs (that is, 1 piece of paper each with a single word on each). Coincidentally, the person who held the sign saying Chris was actually named Chris.