You will need three batches of buttercream to coat the Rose-Petal Wedding Cake.
Put sugar and egg whites in the heatproof bowl of an electric mixer set over a pan of simmering water. Whisk constantly until whites are warm to touch and sugar has dissolved, 3 to 4 minutes (test by rubbing mixture between your fingers).
Transfer bowl to an electric mixer fitted with the whisk attachment. Beat on medium-high speed until stiff peaks form.
Reduce speed to medium-low; add butter by the tablespoon, beating well after each addition. Beat in vanilla and chocolate. Tint buttercream with yellow food coloring, 1 drop at a time, to desired shade. If not using immediately, refrigerate buttercream in an airtight container up to 3 days, or freeze up to 1 month. Bring to room temperature before using; beat on low speed until smooth, about 3 minutes.