I modified a regular recipe and got awesome results on my first attempt at buttercream frosting a layer cake and then topping it with a chocolate glaze.
For the frosting, I took a pound of European-style unsalted butter, let it soften, and blended it with 2 cups of powdered sweetener ( I used a combo of powdered Erythritol and powdered Maltitol) and 1.5 TBSP vanilla. Frost between the layers and then the top/sides of the cake (I used a mix for the cake this time :-)). Refrigerate. While the buttercream frosting is setting up in the refrigerator, place 5 oz of low-carb chocolate chips in a heat-safe bowl (you can finely chop your favorite low-carb chocolate bar as an alternative). Bring 4.5 oz of cream to a full boil, and then immediately pour over the chips. Whisk until smooth. Pour the glaze over the top of the cake so that it completely covers the top with the excess running down the sides. Refrigerate for at least 20 minutes.
P.S. the frosting has 0 net carbs when made with sugar alcohols. The glaze is also extremely low-carb, probably 1/4 to 2 net carbs per serving depending on the chocolate used and the size of the serving.

Where did you find the low carb chocolate chips.

I have to try to remember to get back to you when I'm at home, because I have found a few different brands of them. One is Gol D Lite and another one is also fairly well-known. I find them both online and in low-carb stores. The third one you can get online now at www.carbsmart.com Often they and other sites are out of stock, but after meeting Andew deMino a few months ago he assured me he'd try to keep them in stock because he sees that there is a lot of demand. Sure enough, going to his site I see he is now having them manufactured with his label on it, so that's a good sign that he's keeping em well stocked!
They're terrific--great on top of ice cream and incredible for making brownies, cookies, frostings, etc. Have fun!