Special equipment: a doll dress cake pan; a 4-inch round cutter; a pastry bag with coupler, large star tip, small star tip and small plain round tip
Prepare the cake mix according to the package instructions and bake it in a doll dress cake pan. Let cool for 10 minutes.
While the cake is still in the pan, trim off the top, using the edge of the pan as a guide. This will give you a flat top. Invert the cake onto a cooling rack and let cool completely.
Meanwhile, transfer the frosting to a large bowl and mix with an electric mixer on medium speed, adding a couple of drops of food coloring at a time to achieve the desired shade of blue.
Cut the cake horizontally into 4 layers using a long serrated knife. Use a 4-inch round cutter to cut a hole in the center of the 2 largest layers on the bottom. Reserve the cutouts.
Place the bottom layer on a cake board or cake plate and frost the top with the frosting. Place the second cake layer on top, so that the sides and center holes line up. Fill the well in the cake with the candy-coated chocolates. Frost the top of the second layer, and place the third layer on top. Repeat with the frosting and top cake layer. Using a limited amount of frosting, frost a thin crumb coat over the cake. Refrigerate to firm up, about 30 minutes. Frost the cake with more frosting.
Fit a pastry bag with a coupler (or you can use multiple piping bags). Fit the bag with a large star tip and fill the bag with frosting. (See Cook's Note.)
Use a butter knife to lightly score 5 evenly-spaced vertical lines down the side of the cake. Using the pastry bag with the large star tip and following one of the scored lines from top to bottom, pipe overlapping diagonal 2-inch mounds of frosting, lifting and pulling down in alternating directions to make a braid. Repeat the pattern down the remaining scored lines.
Switch to a smaller star tip on the pastry bag. Working from the top of the cake down, pipe overlapping semi-circles between each of the braided lines. Repeat around the rest of the cake.
Switch to a small plain round tip on the pastry bag. Pipe straight lines down both sides of each semi-circle line. Next to these, pipe lines of dots, starting from the top to the bottom. Next to these, pipe straight lines. Repeat around the cake.
To make the pom-pom on top of the cake, mix the reserved cake cutouts with 1/4 cup frosting in a medium bowl until mostly smooth. Form a cake ball the size of a baseball and refrigerate it on a plate until firm, about 10 minutes.
Place the coconut in a 1-gallon resealable plastic bag and add 2 drops of blue food coloring. Shake the bag to color the coconut. Remove half the coconut to a shallow bowl. Add 6 more drops of the food coloring to the remaining coconut in the bag. Shake the bag to color the coconut and transfer it to a second shallow bowl.
Roll the cake ball in the dark blue coconut followed by the light blue coconut until fully coated. Place the cake ball on top of the cake. Press the remaining coconut around the bottom of the cake to form the trim.
A coupler is a two-part plastic device that lets you swap out piping tips on one pastry bag.
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