2 cups (4 sticks) unsalted butter, room temperature
1 teaspoon pure vanilla extract
Food-coloring gel paste
In the heatproof bowl of an electric mixer, combine sugar and egg whites. Place bowl over pan of simmering water; whisk until whites are hot and sugar is dissolved, 2 to 3 minutes.
Remove from heat, and transfer to mixer fitted with the whisk attachment. Beat on high speed until cooled and fluffy, 10 to 15 minutes. Reduce speed to medium. Add butter 1 tablespoon at a time. Beat until incorporated. Stir in vanilla.
To tint, divide buttercream into small bowls, one for each color of flower petals (setting aside some untinted buttercream, if desired); color about 1/2 cup green for the leaves. Add a small amount of food coloring; stir well. Add a bit more color at a time until desired shade is reached.
Use paste or gel food coloring to tint buttercream; they provide intense hues and don’t cause streaking or flaking (see the Guide). If not using immediately, store frosting, well covered, in the refrigerator up to 1 week, or freeze up to 1 month. Before using, bring to room temperature; beat well.