• Preheat the oven to 350° F. Grease and flour two 9-inch round spring-form pans or cake pans. Cut a round of parchment paper to fit into the bottom of the pan. Grease and flour the parchment paper.
• In a large bowl, whisk together the flour, cornstarch, baking powder and salt. Set aside.
• In the bowl of a large stand mixer, beat the softened butter with the 1½ cups of granulated sugar for about 5 minutes: the mixture should be fluffy and pale yellow in color. With the mixer running on low, slowly pour in the egg whites and vanilla, and mix them into the butter-sugar mixture. Add to the mixing bowl a third of the flour mixture and run the mixer until it is just combined. Add in half of the milk and mix this in too. Repeat these steps with another third of the flour mixture, then the rest of the milk and then finally the last of the flour mixture. The batter should be smooth.
• Divide the batter evenly between two mixing bowls. Add a few drops of red food coloring to one of the bowls. Mix the coloring into the batter. Add more drops as needed until you get a nice shade of pink. Put a few drops of green food coloring into the other bowl of batter and mix the coloring in. Add more green as needed to get the shade you want.
• Transfer the pink batter into one prepared cake pan and the green batter into the other prepared pan. Place the pans in the oven and bake the cakes for 30 minutes. Check them by inserting a toothpick or wooden skewer into the center - it should come out clean. If necessary, bake the cakes a few minutes longer until the tester comes out clean. Remove the pans from the oven and place them on a cooling rack.
• Allow the cakes to cool completely. Remove the cakes from the pans and peel off the parchment rounds. Wrap the cakes in plastic wrap and refrigerate the layers for at least 1 hour or up to two days.

While the cake layers chill, prepare the frosting:

• To prepare the buttercream, beat together the ¾ cup of butter and the ¼ cup of vegetable shortening in the bowl of a stand mixer. Beat in 1 cup of the confectioner's sugar, followed by a little of the milk. Continue alternating the sugar and milk until all of the sugar has been mixed in. Add in the vanilla and beat the frosting until it is light and fluffy. (You may not need all of the milk.) Add in a few drops of blue food coloring and mix it into the buttercream. Add more drops as needed to get a robin’s egg shade of blue.

Assemble the cake:

• Remove the pink cake layer from the fridge and unwrap it. Place it on a round cake platter. Spread about 1 cup of the blue buttercream over the layer and to the edge all the way around. Make the frosting even across the layer with a slight rise around the edge. Remove the green cake layer from the fridge, unwrap it and place it on first layer. Spread a thin layer of buttercream over the top as a crumb coat. Spread a thin layer of frosting as a crumb coat over the sides as well. If there are gaps between the layers, use buttercream to fill them in. Spread the remaining buttercream over the sides and over the top layer, Touch up and fill in gaps along the sides or in places where cake is showing. Make sure the buttercream over the top is smooth.
• Use a toothpick to lightly sketch out the shape of a flower in the buttercream on top of the cake. Use the purple decorating sugar to fill in the center of the flower: use a spoon to sprinkle the purple sugar into the space. When this is done, use a spoon to gently fill in the petals of the flower. As you fill in the spaces use your fingertips or the back of the spoon to gently press the sugar into the frosting.
• Slice the cake and serve. Keep leftover cake in a covered cake storer for up to three days. Serves 10 people.