Preheat your oven to 350°F (180°C). It’s important to get the oven nice and hot before baking to ensure the cake rises properly. Spray 2 (9-inch) round cake pans with baking spray that contains flour to prevent sticking. For extra insurance, line the bottoms with parchment paper as well.
In the bowl of a stand mixer fitted with the paddle attachment, beat unsalted butter, granulated sugar, and orange zest at medium-low speed until combined. You want the mixture to be well-mixed but not overly beaten at this stage. After about 2 minutes, increase the speed to medium and beat until it's fluffy, around 3 to 4 minutes. Remember to stop and scrape the sides of the bowl to ensure everything gets mixed in evenly.
Add the eggs one at a time, beating well after each addition. This will help incorporate air into the batter, making it light and fluffy.
In a separate large bowl, whisk together the cake flour, baking powder, kosher salt, and baking soda. This dry mixture should be well combined to avoid any clumps.
In a medium bowl, whisk together the buttermilk, orange juice, and vanilla extract. This mixture adds moisture and flavor to the cake.
With the mixer on low speed, gradually add the flour mixture to the butter mixture, alternating with the buttermilk mixture. Begin and end with the flour mixture. This method helps keep the batter from becoming too dense. Beat until just combined, stopping to scrape the sides of the bowl as needed.
Divide the batter evenly between the prepared pans. Using a small offset spatula, smooth the tops to create an even surface. To release as many air bubbles as possible, forcefully tap the pans on a kitchen towel-lined counter several times.
Bake in the preheated oven until a wooden pick inserted in the center comes out clean, which should take about 30 to 38 minutes. Keep an eye on them to avoid overbaking.
Once baked, let the cakes cool in the pans for about 10 minutes. Then, carefully remove them from the pans and let them cool completely, parchment side down, on wire racks.
Once the cakes are completely cooled, level the cooled layers and discard the parchment. On a cake turntable or a serving plate, place one cake layer cut side up. Brush it generously with half of the prepared Simple Syrup.
Spoon ½ cup (about 123 grams) of the Creamy Buttermilk Frosting into a pastry bag and cut a ½-inch opening in the tip. Pipe a ½-inch-tall border around the edge of the layer. This will help contain the Orange Cream Filling.
Within the piped border, dollop and spread the Orange Cream Filling. Then, add the remaining cake layer, this time cut side down, and brush it with the remaining Simple Syrup.
Spread a thin layer of frosting over the top and sides of the cake to create a crumb coat. Refrigerate the cake for about 30 minutes to set the frosting.
While the cake is cooling, divide the remaining Creamy Buttermilk Frosting between 2 small bowls. Add food coloring to one bowl until you achieve a deep orange shade and to the second bowl for a light orange. Leave the rest of the frosting white.
Using a small offset spatula, spread the deep orange frosting evenly over the bottom third of the cake sides. Then, spread the light orange frosting over the middle third, and finish with the white frosting on the top and top third of the cake sides. This creates a beautiful gradient effect!
To smooth the frosting, hold a bench scraper or a large offset spatula against the cake sides and scrape off the excess frosting. Use the small offset spatula to create swirls on the top as desired. For a final touch, scrape the sides one more time to achieve a smooth finish. Garnish with orange sprinkles if desired.
