Step 1

Line the bottoms of three, 6-inch cake pans with a round of parchment paper. Preheat the oven to 350 F/180 C.

Step 2

In a stand mixing bowl, combine the flour, sugar, baking powder, and salt.

Step 3

Add the cubed butter to the mixing bowl and mix on low until it looks similar to wet sand, about 2-3 minutes.

Step 4

Mix in the oil, eggs, milk, champagne, sour cream, and vanilla extract until fully combined.

Step 5

Evenly distribute into the cake pans. This is best done with a kitchen scale, each pan should hold about 448 g of batter.

Step 6

Bake the layers on the middle rack of the oven for 28-33 minutes, or until the tops spring back when touched and a toothpick inserted into the center comes out clean.

Step 7

Let the pans cool on a wire rack for 15-20 minutes until cool enough to touch, then knock them out of the pans and let them cool completely on the rack.

Step 8

Next, make the orange juice reduction. Cook the orange juice over medium heat, stirring frequently for 15-17 minutes or until reduced to about ¼ cup. Chill in the fridge until ready to use.

Step 9

Over a double boiler, whisk the egg whites and sugar frequently, until the sugar granules are dissolved, about 8-10 minutes.

Step 10

Transfer the egg mixture to a stand mixing bowl. Using the whisk attachment, whisk on high speed for 7-8 minutes, or until the meringue feels room temperature and holds a stiff peak.

Step 11

Decrease the speed to low, then add the butter one tablespoon at a time.

Step 12

Increase the speed to medium-high and continue mixing. At first, it may look thin and curdled, but that's normal! Keep beating until it turns light and fluffy.

Step 13

Mix in the reduced orange juice and orange zest until combined.

Step 14

To assemble, use a serrated knife to cut a thin layer off the top of each cake layer to expose the crumb.

Step 15

Place a dollop of buttercream on a serving platter or cake stand and place the first layer on top.

Step 16

Evenly spread about ⅓-½ cup of the buttercream on top. Continue with the next layer the same way. For the last layer, place it upside so that the top is completely flat.

Step 17

Set aside a few dollops of buttercream for decorating, then use the rest of it to frost the sides and top of the cake.

Step 18

Tint the reserved buttercream with orange gel food coloring. Then use a spatula or scraper to scrape the orange buttercream randomly on the sides (like paint smears).

Step 19

Enjoy!