Step 1

In a medium-sized bowl, add 1 cup of the granulated sugar, 1 tablespoon of the Meyer lemon zest, 1 tablespoon of the orange zest, and the blood orange zest and, using your clean fingers, rub the zest into the sugar until it is fragrant and the sugar is yellowish-orange in color.

Step 2

Add 3 of the eggs to the sugar mixture and using an electric handheld mixer, beat until completely combined.

Step 3

In a medium-sized saucepan over medium heat, add the egg mixture, the Meyer lemon juice, the orange juice, and the blood orange juice and whisk to combine.

Step 4

Cook the egg mixture, while whisking constantly, until the mixture thickens into a curd, about 10-15 minutes. The curd should be thick enough to coat the back of a wooden spoon.

Step 5

Reduce the heat to low.

Step 6

Add the extra salt and 4 tablespoons of the diced butter to the curd and whisk to combine until the butter is completely melted and incorporated.

Step 7

Transfer the curd to a heatproof bowl or container and place a sheet of plastic wrap directly onto the surface of the curd. This prevents a skin from forming on the curd.

Step 8

Chill the curd for at least 2 hours and up to overnight. Keep the curd refrigerated until you are ready to assemble the cake.

Step 9

Preheat the oven to 350 degrees F.

Step 10

Using the extra butter, grease two 8-inch cake pans, then using the extra flour, lightly dust the greased pans.

Step 11

In a large bowl, add 2 cups of the remaining flour, the baking powder, and 1/2 teaspoon of the salt.

Step 12

In a separate bowl, add the remaining granulated sugar, the remaining Meyer lemon zest, and the remaining orange zest and, using your clean fingers, rub the zest into the sugar until it is very fragrant and the sugar is yellowish-orange in color.

Step 13

Add the sugar mixture to the flour mixture and whisk to combine.

Step 14

Add the remaining eggs, the oil, the lemon extract, the butter flavoring, and the milk to the flour mixture and, using a clean electric handheld mixer on medium speed, beat until just blended into a batter. Do not overmix.

Step 15

Add 4 ounces of the Champagne to the batter and stir to combine.

Step 16

Divide the batter evenly between the two prepared cake pans.

Step 17

Bake until the cakes are golden-brown and are set in the center, about 23-25 minutes.

Step 18

Allow the cakes to cool in the pans for 10 minutes.

Step 19

Transfer the cakes to a wire rack and allow them to cool completely, about 1 hour.

Step 20

In a saucepan over medium heat, add the remaining Champagne and allow it to simmer until reduced to about 1 cup in volume, about 20-30 minutes.

Step 21

Allow the Champagne reduction to cool completely.

Step 22

Cover the Champagne reduction with plastic wrap and store it in the refrigerator until ready to use.

Step 23

In the bowl of a stand mixer with a paddle attachment, add the remaining 1 cup of the room temperature butter and the shortening and beat on medium speed until combined, fluffy, and pale yellow in color.

Step 24

Remove the paddle attachment and replace it with a whisk attachment.

Step 25

Add the heavy cream, 2 tablespoons of the reserved Champagne reduction, and 1/2 of the powdered sugar to the butter mixture and mix on low speed until mostly combined.

Step 26

Scrape the sides of the bowl, as needed.

Step 27

Add the remaining powdered sugar to the butter mixture and mix until it is combined into a fluffy frosting.

Step 28

Using a cake saw or a serrated knife, remove the domed tops of the cake layers. Discard the domed cake tops or reserve them for a separate use.

Step 29

Carefully cut each of the cakes in half horizontally to create 4 thin cake layers.

Step 30

Using a pastry brush, brush the tops of each of the cake layers with the remaining Champagne reduction.

Step 31

Place 1 of the cake layers on a serving plate or a cake stand.

Step 32

Top the cake layer on the serving plate with a thin layer of the frosting.

Step 33

Using a piping bag, pipe a border of frosting around the top of the cake layer on the serving plate. This is critical for keeping the curd from escaping the cake layers.

Step 34

Transfer the curd from the refrigerator and stir or whisk it.

Step 35

Using a tablespoon, spoon 2-3 tablespoonfuls of the curd on top of the cake layer on the serving plate and spread it out evenly to the edges of the frosting border.

Step 36

Add 1 of the remaining cake layers on top of the curd layer. Top the cake layer with a thin layer of the frosting, then using a piping bag, pipe a border of frosting around the top of the cake layer. Spoon 2-3 tablespoonfuls of the curd on top of the cake layer and spread it out evenly to the edges of the frosting border. Repeat these processes with 1 of the remaining cake layers.

Step 37

Add the remaining cake layer on top of the curd layer.

Step 38

Add the remaining frosting to the top of the cake and the outer edges, spreading it out until smooth and the entire cake is coated in the frosting.

Step 39

Garnish the cake with the orange slices.

Step 40

Serve.