I love the cookie dough cupcakes and now use that mix base for all of my yellow cakes minus the cookie dough.
The buttercream recipes that I've tried are all amazing. I especially like the lemon buttercream (very lemony!) and chocolate buttercream.
I tried the blackout cupcakes (with a different frosting than recommended in the book) and found them to be very dry. The second time I made them I added some extra oil but they were still dry.
I've made the mini lemon cupcakes a few time. I make homemade lemon curd and find the flavor of these cupcakes (with only 1/3 cup lemon curd) to be very subtle. Don't use it with the lemon buttercream though, it tends to be a bit overwhelming in combination.