Preheat the oven to 325°F. Grease and flour the cake pans. Cut parchment paper circles and place them in the prepared pans. Grease and flour the parchment circles.
In a large mixing bowl, whisk together the lemon zest and juice, buttermilk, sour cream, vanilla, eggs, egg white, and oil until smooth and well blended.
Sift the cake mix into the wet ingredients. Stir just until blended. Do not overmix.
Divide the batter between the prepared cake pans. Bake on the center rack for 26-28 minutes. Insert a toothpick to check for doneness. If it comes out clean or with crumbs, the cake is done. If not, bake for a few additional minutes and check it again with a toothpick.
After baking, let cakes cool in the pans for 5-10 minutes. Run a knife around the edges and flip onto a cooling rack. Remove the parchment paper circles and cool completely.
Buttercream
In a mixing bowl, beat the butter until smooth.
Sift sugar and salt together, then add it to the butter one cup at a time, beating until smooth, scraping down the sides as needed.
Add the elderflower syrup, lemon juice, and vanilla extract and beat until thoroughly mixed for about 5 minutes.
Frost each layer, taking time to smooth the frosting as you go. Use a piping bag fitted with a star tip to pipe buttercream stars around the bottom of the cake.
Decorations
Wash and dry the edible flowers, then set them aside.
Cut the lemon into slices and set aside.
Cut flower stems to the desired length. Shorter stems work best.
Heat the chocolate chips in a microwave-safe bowl for 30-second intervals, stirring after each, until melted. Dip the flower stems in the chocolate to seal them; let them dry.
Place flowers and lemon slices on the cake in a pleasing arrangement.
Notes
Cake Doneness Tests
You can test your cake for doneness by inserting a toothpick or cake tester into the center. If it comes out with batter, bake it a little longer. When finished cooking, the toothpick will come out clean or with crumbs.Another way to test a cake's doneness is to press on the center and then release. If the center bounces back into place, the cake is done.