New Year's Champagne Cake

Ingredients

• 1 cup granulated sugar
• 1 tablespoon Meyer lemon zest
• 1 tablespoon orange zest
• 1 tablespoon blood orange zest
• 3 large eggs
• 1/3 cup fresh Meyer lemon juice
• 1/3 cup fresh orange juice
• 1/3 cup fresh blood orange juice
• salt, to taste
• 4 tablespoons unsalted butter, diced
• butter, to taste, for greasing the cake pans
• 2 cups all-purpose flour, plus more, to taste, for dusting the cake pans
• 1 tablespoon baking powder
• 1/2 teaspoon salt
• 1 1/2 cups granulated sugar
• 1 tablespoon orange or blood orange zest
• 1/2 cup vegetable oil
• 1 teaspoon lemon extract
• 1 teaspoon butter flavoring, optional
• 3/4 cup milk
• 4 ounces Champagne (from a 750ml bottle)
• 21 1/4 ounces Champagne (the remaining Champagne from the bottle used for the cake)
• 1 cup butter, room temperature
• 1 cup shortening
• 2 tablespoons heavy cream
• 4 cups powdered sugar
• orange slices, optional, to taste, for garnish