CHAMPAGNE REDUCTION: bring champagne to a simmer over medium heat and let simmer until reduced to 1 cup. Let cool completely before using.
CHAMPAGNE CAKE: Preheat oven to 350°F and spray three 6-inch cake pans with baking spray.
Whisk together flour, baking powder, and salt. Set aside. Combine sugar, oil, and vanilla in a large bowl and mix on medium speed with an electric mixer until smooth. Add eggs and mix well. Add half of flour mixture and half of champagne. Mix until just combined. Add remaining flour, champagne, and the milk. Mix well. Stir in a couple drops of food coloring, or until desired color is reached.
Divide batter evenly among cake pans. Bake 20-25 minutes or until a toothpick inserted into the center of each cake layer comes out clean. Let cakes cool in pans for 5-10 minutes, then transfer to a wire rack to cool completely before frosting.
CHAMPAGNE BUTTERCREAM: Cream butter on medium-high speed until smooth and creamy. Add shortening and mix well. Add champagne and half of powdered sugar. Mix on low speed until just combined, and then medium-high speed until completely combined. Repeat with remaining powdered sugar. Spread frosting over and around cake. Decorate with macarons and fresh flowers, if desired.