Step 1

In a medium heavy-bottom pot, whisk together the sugar, flour, and cornstarch. Whisk in the orange juice and egg yolks until well combined.

Step 2

Set over medium-high heat and cook, stirring constantly with a whisk, until the mixture boils, about 5 minutes

Step 3

Continue stirring constantly, until the mixture will become thick and clear, and begins to bubble. Allow mixture to boil for 1 minute.

Step 4

Immediately remove from the heat and stir in the cubed butter. Transfer the filling to a bowl. Press plastic wrap directly onto the surface of the filling, and chill until cold, about 2 hours.

Step 5

Position 2 oven racks towards the center fo the oven. Preheat the oven to 350°F. Butter and flour 3 (8-ch) round cake pans or spray with baking spray.

Step 6

Sift together the flour, baking powder, and salt into a large bowl.

Step 7

In a large mixing bowl or the bowl of a stand mixer fitted with a paddle attachment, beat together the butter, sour cream, and orange zest on medium speed until well combined, about 1 minute.

Step 8

With the mixer running, slowly add the sugar. Increase the speed to high and beat until the mixture is light and fluffy, about 2 minutes. Scrape down the sides of the bowl. Add the eggs, one at a time, mixing well and scraping down the sides between additions.

Step 9

With the mixer on low speed, add the flour mixture in three additions alternating with the orange juice to the butter mixture. Divide the batter among the prepared cake pans. (I also use cake strips on my pans for more even baking.)

Step 10

Bake for 30 minutes until a toothpick inserted in the middle comes out clean, rotating the cake pan positions after 20 minutes. Cool the cakes in the pans for 10 minutes, then loosen the layers by running a knife between the cake and the edge of the pan. Invert the cakes onto a wire cooling and cool completely.

Step 11

In a medium saucepan, combine 2⅔ cups sugar and the water. Place over medium-high heat. Stir constantly until sugar melts and becomes clear.

Step 12

Bring to a boil. Insert a candy thermometer and boil, without stirring, until the temperature reaches 235°-240°F, about 8 minutes.

Step 13

While the syrup boils, combine the egg whites and the remaining 1/3 cup of sugar in a large mixing bowl or the bowl of a stand mixer fitted with the whisk attached. Beat on medium-high speed until soft peaks form.

Step 14

When the sugar syrup comes to temperature, carefully drizzle it into the egg whites while the mixer is running on medium speed.

Step 15

Once combined, add the salt an increase the speed back to medium-high. Continue beating until the meringue is no longer warm to the touch, about 10 minutes.

Step 16

With the mixer running, add the butter into the meringue, one piece at a time. Beat until the butter is combined and the frosting is smooth but stiff enough to spread. Beat in the vanilla.

Step 17

If desired, divide the buttercream into a few batches and added varying amounts of food coloring to create an ombre effect. Transfer about 1 cup of the frosting to a piping bag fitted with an 869 or other decorative piping tip and about 1 cup in a piping bag with about ½ inch snipped off the end.

Step 18

Place one cooled cake layer one a cake stand or plate. Using the frosting in the piping bag without the tip, pipe a single ring around the top edge of the cake creating a frosting barrier. Spread ½ of the Filling over the cake to the frosting barrier. Top with another cake layer. Repeat with more frosting and the remaining filling. Place the remaining cake layer on top.

Step 19

Spread the remaining frosting on the outside of the cake in the desired pattern. Using the decorative piping tip, piping frosting on top of the cake. Refrigerate the cake for at least 30 minutes to set the frosting before slicing.