Special equipment: a 12-cup standard muffin tin, two 9-inch round cake pans, 4 pastry bags, each fitted with a 1 1/16-inch plain round tip (number 809)
For the cake: Position the oven racks in the upper and lower third of the oven and preheat to 350 degrees F. Line 3 cups in a 12-cup standard muffin tin with cupcake liners. Grease the bottoms and sides of two 9-inch round cake pans.
Prepare the cake batter according to the package instructions. Spoon 1/4 cup of the batter into each of the lined muffin cups (they should be about three-quarters full). Divide the remaining batter evenly between the prepared cake pans (about 2 cups batter per cake pan).
Bake the cupcakes and the cake, rotating the pans about halfway through, until a toothpick inserted into the center of the cupcakes and cake comes out clean, 16 to 18 minutes for the cupcakes and 18 to 20 minutes for the cakes. Let cool in the pans about 10 minutes, then turn out onto cooling racks and let cool completely.
For the frosting: Meanwhile, combine the butter, confectioners' sugar, milk, vanilla and salt in the bowl of a stand mixer fitted with the paddle attachment (or in a large bowl if using a handheld electric mixer). Mix on low speed until mostly incorporated. Increase the speed to medium-high and mix until smooth.
Tint 1 heaping cup of the frosting with the green gel food coloring in a medium bowl. Scrape the green frosting into a pastry bag fitted with a 11/16-inch plain round tip (number 809).
Tint 1 heaping cup of the frosting with 24 drops Christmas red gel food coloring in another medium bowl. Scrape the dark red frosting into a pastry bag fitted with a 11/16-inch plain round tip (number 809).
Tint 1 heaping cup frosting with 10 drops of Christmas red gel food coloring in a third medium bowl. Scrape the light red frosting into a pastry bag fitted with a 11/16-inch plain round tip (number 809).
Tint the remaining frosting (about 3 cups) with the pink gel food coloring and 2 drops Christmas red gel food coloring. Scrape 1 1/2 cups of the pink frosting into a pastry bag fitted with a 11/16-inch plain round tip (number 809). Cover the remaining pink frosting with plastic and reserve for layering the cake.
To assemble: Place 1 cake layer upside down on the top half of on an extra-large cutting board or platter (at least 8 1/2-by-9 inches) and spread with the reserved pink frosting. Top with the second cake layer.
Using a serrated knife, cut the cake in half across the center. Make a second cut that crisscrosses the first so that the 2 cuts form a narrow X, with the top half of the X slightly wider than the bottom. You will have 2 small wedges and 2 large wedges; the cuts should intersect slightly below the center of the cake.
Remove the small wedge cut out of the top of the cake. Trim off the larger two corners to make it into a diamond shape. Set the trimmed corners aside. Snuggle the diamond back in between the larger two wedges.
For the ombre frosting: Pipe a thick line of dark red frosting on the large left wedge in a crescent shape along the outer edge. Pipe a thick line of dark red frosting on the large right wedge in a crescent shape along the upper portion of the inner edge, stopping where the four pieces of cake come together in the center (do not let the line run over on the left wedge). Pipe a thick border of dark red frosting around the diamond in the middle of the cake.
Pipe a thick line of light red frosting on the large left wedge next to the dark red frosting. Pipe 2 thick lines of light red frosting in on the large right wedge next to the dark red frosting to continue filling in the wedge. Pipe a line of light red frosting into the center of the diamond to fill in the diamond.
Pipe thick lines of the pink frosting in crescent shapes next to the light red frosting on both large wedges so the cake is fully covered.
Carefully drag a small offset spatula from the top of the left large wedge to the bottom, curving with the lines to smooth and blend the frosting without mixing up the colors. Wipe the spatula clean and repeat on the right side. Wipe the spatula clean and use it to smooth out the frosting in the diamond.
Arrange the cupcakes in a line at the bottom of the tulip to create a stem. Pipe the green frosting in three straight vertical lines to cover and coat the cupcakes. Arrange the reserved cake corners along the stem to create leaves and pipe the green frosting over the top to cover. Carefully drag a small offset spatula from top to bottom to smooth the frosting on the stem and leaves.
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