Combine water and sugar in a large skillet and bring to a boil, whisking frequently until sugar has dissolved. Add the orange slices carefully, in a single layer, and reduce heat to a simmer. Simmer orange slices uncovered for about 45 minutes.

Place a wire cooling rack on top of a cookie sheet. Remove oranges from sugar water and place on the cooling rack to dry. Allow to dry, uncovered, overnight. Sprinkle with sugar if desired.

Preheat oven to 350 degrees Fahrenheit and prepare three six-inch cake pans by lining the bottoms with parchment paper. Then, generously butter and flour the pans.

Whisk together the cake flour, baking powder, baking soda, salt, and poppy seeds in a medium-sized bowl. Set aside.

Using an electric mixer, cream together the butter and sugar for 3-4 minutes until light and fluffy. With the mixer on low speed, add eggs, then egg yolk, one at a time. Mix until just combined after each addition. Add orange zest and juice, then mix to combine.

Turn mixer to low speed and alternate adding 1/3 of the flour mixture then 1/2 of the buttermilk, starting and ending with the flour. Mix until incorporated after each addition.

Divide batter evenly among the three cake pans. Bake at 350 degrees for 25-30 minutes, or until cakes pass the toothpick test. Let cool in pans for 5-10 minutes, then turn cakes out onto a wire rack to cool completely.

Add egg whites and granulated sugar to the bowl of a double boiler.*** Whisk to combine, then turn heat to medium. Whisking frequently, continue to heat the mixture until sugar has dissolved and mixture feels smooth when rubbed between two fingers. If you like, use a candy thermometer - it's ready when it reaches about 160 degrees Fahrenheit.

Pour mixture into bowl of a stand mixer fitted with the whisk attachment. Beat on high speed for about 8-10 minutes until stiff peaks form and the mixture has cooled to room temperature. Once cooled, switch to the paddle attachment and reduce speed to medium-low. Add butter a tablespoon at a time, beating until incorporated.

Once the butter is mixed in, add the orange juice and zest and beat on medium speed until smooth. If desired, add gel food coloring one drop at a time until desired color is reached.

Once cakes are cool, level each layer using a serrated knife. Use a small amount of buttercream to secure the first layer on top of a cake board. Top with a layer of frosting, then repeat with the second layer. Top with third cake layer, cut side down. Frost top and sides of the cake with a thin layer of buttercream, then chill the cake for about 20 minutes.

Finish frosting the cake with the remaining buttercream, using an offset spatula and cake scraper to smooth the sides. Decorate with candied orange slices as desired.