Crayons/Dollygirl's birthday cake this year was a Small Chocolate Cake frosted with "Lorna's Super Chocolate Chip Icing" from Edna Staebler's More Food That Really Schmecks. The amount given here will frost a layer cake reasonably, or a one-pan cake extravagantly. We went with extravagantly. But you could probably cut the recipe in half.
I put some of the icing into a decorating tube and made ruffles along the edges, along with a few powdered-sugar butterflies. How do you make powdered-sugar butterflies? Sink a small butterfly-shaped cookie cutter into the icing (that is, the top of the cake that's already been iced), carefully spoon a bit of powdered sugar inside the "walls" of the cutter, smooth it down, then gently lift up the cutter. Repeat several times in other places on the cake--we had five butterflies. I used the icing in the tube to give them chocolate bodies and antennae (which also helped show what they were supposed to be!), and a few dots on their wings.
And one warning about that, if you're blowing out candles--don't blow too enthusiastically or you'll have powdered sugar everywhere.
Lorna's Super Chocolate Chip Icing
6 oz. semi-sweet chocolate pieces (chips)
1/2 cup light cream (I used 2% milk)
1 cup margarine or butter (I used butter)
2 1/2 cups icing sugar (powdered sugar)
a bowl full of ice cubes plus some water (this doesn't go into the icing, it's for a quick cooling off)
In a saucepan combine the chocolate, cream/milk and butter; stir till smooth, remove from the heat, and whisk in the sugar--it will be thin. Beat it over ice until it holds its shape (put ice cubes and water in a bowl, and rest the saucepan on top. I put the bowl in the sink.). Unless you have a very strong whisk, I'd recommend using a wooden spoon for this part--the icing will get thick.
A shortcut: in the book, Edna says that she didn't have time to stand around beating the icing, so she just put it in the fridge to set. I have done it that way, but this time I beat it over the ice, and it turned out really well.