Tint buttercream with pink food coloring to desired shade; set aside. Place coconut in a resealable plastic bag. In a small bowl, combine water and petal dust, and stir until petal dust dissolves. Add to coconut. Seal bag, and shake well to color coconut evenly; set aside.
Using a serrated knife, trim top of cake level then split horizontally, forming two layers. Place cake layers on a cutting board. With offset spatula, smooth 1 1/2 cups buttercream over one layer. Top with second layer, pressing lightly to adhere. Cut crosswise into three equal rectangles (9 by 4 inches). Smooth 1/2 cup buttercream over top of one rectangle. Top with a second rectangle, pressing lightly. Smooth 1/2 cup buttercream over top of second rectangle. Top with the third rectangle, forming a cake with six layers.
Measure 1 1/2 inches from one of the short sides of the cake, and place a long skewer through the cake at this point. Repeat on the second short side (this will help hold cake in place while cutting and ensure the cake is trimmed evenly). Position cake with one the long side facing edge of counter. Starting at the skewers, use a serrated knife to trim away sides of cake at an angle, forming a purse shape; the widest part at the bottom should measure about 9 inches and the narrowest part at the top should measure about 6 inches. Thinly coat entire cake in buttercream. Refrigerate until firm, about 30 minutes.
Smoothly coat cake with remaining buttercream, using offset spatula. Pat colored coconut evenly over surface of cake.
Use a trussing needle to poke a hole through the center of each malted milk ball. String the malted milk balls onto an 18-inch length of cloth-wrapped floral wire. Bend into an arc, and stick ends into top of cake to form the handle. Stick a toothpick into the back of the chocolate. Stick toothpick into cake, centered on front about 2 inches from top, to secure the heart to the cake.