Step 4: Pipe That Icing
After you’ve added the sprinkles, it’s time to pipe the rest of the cake. I’d suggest putting your icing into a piping bag and cutting off the tip. Then pipe where the frosting needs to go: either side of the sprinkle line. Make this layer fairly thick so that the fault line stands out. Next, using the bench scraper and/or offset spatula, smooth the frosting until you have a uniform finish.