Starting w/a cake that has a crumb coat. I put a acrylic disc on top to help with the sides. Temperature is key to stripes. The cake can’t be too cold and the room needs to be 68-70 degrees. The disc helps with smooth straight sides. You don’t have to have it but it does help. To make black buttercream I make chocolate buttercream and add black gel coloring. These are the cutest sprinkles ever. They are from Sprinkle pop!