For the Filling:

1. In a medium heavy-bottom pot, whisk together sugar, flour, and cornstarch. Add orange juice and egg yolks, whisking until smooth.

2. Cook over medium-high heat, stirring constantly, until the mixture reaches a boil (about 5 minutes).

3. Continue stirring until the mixture thickens and turns translucent. Let it boil for 1 minute.

4. Remove from heat immediately and stir in cubed butter.

5. Transfer to a bowl, cover with plastic wrap (pressing it directly onto the surface), and refrigerate for 2 hours.

For the Cake:

1. Preheat oven to 350°F (175°C). Butter and flour three 8-inch round cake pans or coat with baking spray.

2. In a large bowl, sift together flour, baking powder, and salt.

3. In a mixing bowl, beat butter, sour cream, and orange zest on medium speed until smooth (about 1 minute).

4. Gradually add sugar while mixing. Increase speed to high and beat until fluffy (about 2 minutes). Scrape down the sides.

5. Add eggs one at a time, mixing well and scraping the bowl between additions.

6. On low speed, alternate adding the flour mixture in three parts with the orange juice, mixing just until combined.

7. Divide batter evenly among prepared cake pans. (Cake strips can be used for even baking.)

8. Bake for 30 minutes, rotating pans after 20 minutes, until a toothpick inserted in the center comes out clean.

9. Cool in pans for 10 minutes, then run a knife along the edges and invert onto a wire rack to cool completely.

For the Buttercream:

1. In a medium saucepan, combine 2⅔ cups sugar and water. Heat over medium-high, stirring until the sugar dissolves.

2. Bring to a boil and cook without stirring until the syrup reaches 235°-240°F (soft-ball stage), about 8 minutes.

3. Meanwhile, in a stand mixer, beat egg whites and remaining ⅓ cup sugar on medium-high until soft peaks form.

4. With the mixer running on medium, slowly drizzle in the hot syrup.

5. Once combined, add salt and increase speed to medium-high. Beat until the meringue is cool to the touch (about 10 minutes).

6. With the mixer running, add butter one piece at a time, beating until smooth. Mix in vanilla.

7. For an ombre effect, divide the buttercream and tint with varying amounts of orange food coloring.

8. Transfer about 1 cup to a piping bag fitted with a decorative tip and another cup to a bag with a ½-inch opening.

For Assembly:

1. Place one cake layer on a stand or plate. Using the plain piping bag, pipe a frosting ring around the edge to create a barrier.

2. Spread half the filling inside the ring, then top with another cake layer.

3. Repeat with more frosting and the remaining filling. Place the final cake layer on top.

4. Frost the cake as desired. Use the decorative piping tip for finishing touches.

5. Refrigerate for at least 30 minutes to set the frosting before slicing.

Enjoy your beautifully layered orange cake! 🍊🎂