I'll admit that I spent a hot second Googling to see if I could make a homemade version (you absolutely can!) but ultimately I decided that I wasn't going to risk ruining someone's 16th birthday for the sake of my own compulsions. Cake mix it was.
I did, however, allow myself some room for interpretation on the frosting.
Let's talk buttercreams. Standard American buttercream is just butter and powdered sugar, beaten together like crazy. There are people out there (I live with one) who feel that American buttercream is too sweet. Luckily there are plenty of other options. There is Swiss meringue buttercream, which I used for my sister's wedding cake. It uses egg whites cooked with sugar and piles and piles of butter. It is beautiful and glossy and people who hate American buttercream will often love it. I'm also a pretty big fan of the old school cooked flour frosting. I used French buttercream (egg yolks cooked with sugar) on the Barbie cakes with great success. But why do something you've already tried when you have a chance to make something new?
German buttercream starts with a pudding base and then you whip in plenty of butter, giving you a fluffy frosting with a texture similar to whipped cream and an almost ice cream like flavor. I used the recipe from BraveTart, my favorite place for professional dessert advice. As usual, she didn't let me down. This frosting was so, so good and it held up beautifully. One kid asked for seconds on the frosting. No cake. I mean, sure, maybe he's equally enthusiastic about Costco frosting but D is definitely not and even he kept nicking spoonfuls from the bowl.
As always, I started with a crumb coat and refrigerated till firm. Then I mixed a few handfuls of sprinkles into the remaining frosting (to simulate the Funfetti effect of canned frosting) and spackled on the outer coat with an offset spatula. To decorate, I just added a layer of sprinkles around the bottom edge of the cake and then a small burst in the center. Done and done.