How to Make It
Prepare the Cake Layers: Place white chocolate in a bowl. Pour boiling water over chocolate, and let stand 1 minute; stir until chocolate is melted and smooth. Cool completely.
Preheat oven to 350°F. Beat granulated sugar and butter with a heavy-duty stand mixer on medium speed until fluffy, about 5 minutes. Gradually add egg whites, a third at a time, beating well after each addition.
Sift together flour, baking powder, and salt; add to butter mixture alternately with white chocolate mixture, beginning and ending with flour mixture. Beat on low speed until blended after each addition. Add vanilla bean paste. Pour into 3 greased (with shortening) and floured 9-inch round cake pans.
Bake in preheated oven until a wooden pick inserted in center comes out clean, 22 to 25 minutes. Cool in pans on wire racks 10 minutes; remove from pans to wire racks, and cool completely.
Prepare the Mousse Filling: Microwave white chocolate chips and 1/4 cup of the cream in a microwave-safe glass bowl on MEDIUM (50% power) until melted and smooth, 1 1/2 to 2 minutes, stirring every 30 seconds. Stir in crème de cacao. Cool 5 minutes.
Beat remaining 1 cup cream with an electric mixer on medium speed until soft peaks form; fold into white chocolate mixture. Chill in refrigerator.
Prepare the Frosting: Microwave chopped chocolate and 1/3 cup heavy cream in a microwave-safe bowl on MEDIUM (50% power) until melted and smooth, 1 to 1 1/2 minutes, stirring every 30 seconds. (Do not overheat.) Cool completely.
Beat butter with an electric mixer on medium speed until creamy; gradually add powdered sugar, beating on low speed until blended after each addition. Add 1/4 cup heavy cream, 1 tablespoon at a time, beating on low until blended after each addition. Add salt and white chocolate mixture, and beat on low until combined; increase speed to high, and beat until light and fluffy. Stir in vanilla.
Assemble the Cake: Place 1 Cake Layer on a serving plate. Pipe a 1/2-inch-thick ring of Frosting around edge of Cake Layer. Spread half of Mousse Filling inside Frosting border; top with 1 Cake Layer. Pipe a 1/2-inch ring of Frosting around edge of Cake Layer. Spread remaining Filling inside border. Place remaining Cake Layer on top; spread top and sides of cake with remaining Frosting. Garnish with Poinsettia Cookies and Royal Icing as directed below in "Garnish How-To."
Make the Poinsettia Cookies: Roll out 1 (5-oz.) pkg. refrigerated sugar cookie dough to 1/8-inch thickness. Cut the dough into 12 large petals with a 3-inch leaf cookie cutter. Cut 3 small petals from dough, using a 2-inch leaf cookie cutter and the center of the poinsettia using a 1 1/2-inch flower cutter. Bake at 350°for 7 to 9 minutes. Decorate the cookies: To make Royal Icing, beat 16 oz. powdered sugar, 2 tsp. meringue powder, and 5 to 6 Tbsp. warm water with an electric mixer on high speed, 5 minutes. Pipe the icing onto cooled cookies. Top the center poinsettia cookie with gold dragées, if desired. (Remove before eating.) Let the cookies dry. Use a paintbrush to paint the cookies with edible ivory luster dust and gold luster dust. Pipe the cake: Make another batch of Royal Icing. Spoon it into a ziplock plastic bag, and snip 1 corner of the bag. Pipe a column of 4 (1-inch) dots of frosting down the side of the frosted cake. (The dots should touch.) Place a 1-inch-wide spatula on the center of the first dot, and drag the frosting outward, creating a petal-like shape. Repeat with the remaining 3 dots, wiping the spatula clean. Pipe a second column of dots on top of the swiped edges, and drag the frosting outward. Repeat, covering the cake, ending with 1 column of dots. Embellish the cake: Arrange the cookies in a flower shape on top of the frosted cake.