dependable and delicious ganache (which i use to make truffles) uses real chocolate and has a chocolate to cream ratio of 2:1 (example: for 350g or 12 oz bag of semi-sweet chocolate chips you'd use 6 oz or 165ml (that's about 1/2 cup plus 3 tablespoons) of 35% whipping cream -- you can also use liquid whip topping for a lactose-free version).
put your chocolate chips (or finely chopped chocolate) in a glass or metal bowl. heat your cream till just boiling in a small saucepan (keep an eye on it as it boils over quick), and pour the hot cream over the chocolate. cover the bowl in plastic wrap and let sit for a couple of minutes, remove wrap and stir with metal utensil (never use wood); you can flavour it at this point with liquers if you want to. cool the mixture at room temperature till cooler, and then whip with your mixer for several minutes. if the cake isn't to be served in the few hours after preparation, you should refrigerate it, but let it sit out a good hour before serving, since the flavour of chocolate depends on its temperature (how fast the fats in the chocolate melt in the mouth - and chocolate has a melting temperature ideal for human mouths!
, but that's also why frozen real chocolate doesn't have the same flavour unless it's allowed to melt away on the tongue).
an alternative to both ganache or buttercream that i use a lot is to make an egg-free chocolate mousse that sets up quite firm; it's great for both filling and frosting (adults really like this instead of sugary buttercream, and it's lighter than ganache). first i melt about 350g of chocolate (in a glass dish) with about 1 tbsp of butter or margarine in the microvave for 2 minutes at 50% power. stir, and let cool. meanwhile, whip 500ml (2 cups - i think that's a pint in the US) 35% whipping cream or nutriwhip as stiff as you can without turning it to butter. add a small amount of the whipped cream to the cooled chocolate, and mix it till smooth, and then add more cream (basically, you're slackening off the chocolate so it'll combine more smoothly with the larger portion of cream). finally, add the chocolate cream to the whipped cream and mix till it's all combined. use to fill or frost (and it's also delicious frozen!). it will be soft at first, but will firm up on standing or even more so with refridgeration.
have a chocolatey good time, emi!