Place the bowl over a pan of simmering water. Make sure the water does not touch the bowl.
Whisk the egg whites occasionally while it warms up. When the mixture has warmed, whisk it constantly. You will heat the mixture until it reaches an internal temperature of 160ºF or until it's not grainy between your fingers.
Transfer the bowl to a stand mixer, mix with a whisk attachment until you get room temperature glossy peaks.
Switch to a paddle attachment and while the mixture is running on low, add tablespoon sized dollops of room temperature butter making sure to let the butter incorporate before you add the next piece. Once all the butter is incorporated, go ahead and add the vanilla extract. You can transfer it to a piping bag or to a ziplock bag to store in the freezer.
To Assemble
Divide your buttercream into batches and tint with food coloring.
Choose a Russian piping tip to use and place in a piping bag with the tip snipped off.
Use a small spatula or knife apply a layer of buttercream to the inside of the piping bag.
Fill the bag with an other color of buttercream.
Smear a thin layer of buttercream onto a cupcake or the surface of your cake. This will allow the piped flowers to adhere to the surface.
Pipe the flowers. If you're using a soft meringue-based buttercream make sure to pipe just above the surface, squeeze and the buttercream will come down to make contact with the cake, Continue squeezing while you pull up to build the flower. Finish by pulling up without squeezing. You can pipe leaves to fill in the blank spaces between the flowers.