This technique looks quite challenging, but trust me once you get the hang on if, it is actually pretty easy and you can achieve beautiful smooth lines on any kind of frosting/buttercream cake.
Tools you will need:
Crumb Coated Cake
1-Apply a generous amount of whipped chocolate ganache to your crumb coated and chilled cake.
2-Get it as smooth as possible on all sides and push the lip of frosting at the top towards the center.
3-Chill your frosted cake for 20 minutes so the ganache hardens a little.
4-Take your cake out of the fridge and using a striped cake comb, gently turn the cake while pushing the comb, removing the excess frosting and creating the stripes lines. The comb needs to be straights and standing leveled to the cake. Keep you cake comb still while turning the cake with the other hand.
5-Using a piping bag with round piping nozzle, or a sandwich bag with a round whole in the corner, fill the stripes with your frosting colors of choice, here I chose to do rainbow stripes, so for each striped I will apply one color of the rainbow. For this frosting I used vanilla Swiss Meringue Buttercream.
6-Take your frosting smoother and gently line it up with the cake. Keep it still while turning the cake with your other hand, removing the excess frosting. It will look messy at first, but keep turning, and with each turn the lines will appear smoother and neater. It is important to have the first frosting of the cake chilled at this point so it does not mix with the stripes.
7-Now you have smooth and clean stripes on your cake, you can finish decorating it however you like.
I chose to add a chocolate ganache drip, some chocolate frosting on the top with homemade French Macarons and sprinkles.
Make sure to send me a photo/comment below with your striped cakes when you try this technique ;)