You will need approximately two times this recipe for filling and covering the tiers of the Chocolate Petal Cake. We used Guanaja 70 percent bittersweet chocolate by Callebaut for its deep flavor and dark color.
Place chopped chocolate in a heatproof bowl or the top of a double boiler. Set over pan of simmering water. Stir until melted, remove from heat, and stir occasionally until cooled. Set aside.
Combine sugar and egg whites in the bowl of an electric mixer. Place the bowl over a pan of simmering water; whisk until the mixture feels hot to touch and the sugar is completely dissolved, about 3 minutes. Transfer the bowl to the mixer stand. Using the whisk attachment, beat the mixture on medium-high until cooled, about 15 minutes.
Add butter pieces, one at a time, stirring constantly until incorporated. Add melted, cooled chocolate, and stir well. Use the buttercream immediately, or transfer to airtight container, and refrigerate up to one week. Bring the buttercream to room temperature, and stir with rubber spatula before using.