Preheat oven to 350℉ and position the rack in the middle of the oven. Line the bottom of two 9” round cake pans with parchment paper, spray the sides of pans with non-stick cooking spray, and set aside.

In a large mixing bowl, beat the egg whites with a hand mixer on high until stiff peaks form. The egg whites will be at stiff peaks when they become very glossy, and the peaks of egg whites will not flop forward when you turn the beater upside down. 

In a second large bowl, combine the remaining cake ingredients with a hand mixer on medium, mixing for 2 minutes. Take a third of the egg whites and gently fold them into the batter with a silicone spatula until just combined. Gently fold in the remaining egg whites, do not stir as it will deflate them. It’s ok if a few lumps of egg white are visible. 

Divide the batter evenly between the two prepared pans. Bake for about 24 minutes, or until a toothpick inserted in the center of the cake comes out clean and the cake begins to pull away from the sides of the pan. Begin checking the cakes after about 18 minutes. Once fully baked, remove them from oven and let them cool in the pan for 10 minutes. Remove cakes from pan and wrap tightly with plastic wrap, refrigerate on a flat surface until completely cold, about 1-2 hours.

Make the frosting: In a large bowl, beat the butter on medium speed until creamy, add marshmallow fluff and lemon zest and beat on medium-low until just combined. Alternate adding ½ cup confectioners sugar and a splash of lemonade, mixing on low speed, until the frosting is the consistency of canned frosting. Transfer to a large piping bag fitted with any large tip, or a gallon zip-top bag with the corner snipped off.

Unwrap the cakes and place one cake, parchment paper side down, on a plate or cake stand. Take a large serrated knife and level the cake by trimming the “dome” off the cake. Pipe a ¾” high bead of frosting around the outer edge of the bottom cake. Fill the well with lemon curd and smooth out with a spoon. Place the second cake on top and press firmly. Pipe remaining frosting generously on top of the cake, and sides if desired, and smooth with an offset spatula or the back of a large knife. Decorate with lemon slices and edible flowers, if desired. Chill the cake for at least 2 hours before serving.