Elderflower blooms are difficult to procure in the United States, but elderflower liqueur imparts the same perfumed flavor and is widely available. It is used in this recipe to marvelous effect.
Make the cake layers: Preheat the oven to 350°F. Grease and flour two 8-inch round cake pans.
Whisk together the flour, sugar, baking powder and salt in the bowl of an electric mixer fitted with the paddle attachment. Turn the mixer on low speed and add the cold butter one cube at a time. After all of the butter has been added, let the mixer run on low speed until the flour mixture resembles fine cornmeal, about 5-7 minutes.
With the mixer still running, add half of the milk and the lemon extract. Increase speed to medium; mixture will be thick. Add the remaining milk and egg whites. Beat until just combined on medium speed. Stop the mixer and scrape down the edges of the bowl. Fold batter with a rubber spatula until the mixture is consistent with no streaks or pockets of flour remaining. Divide batter between prepared pans. Bake for 30-35 minutes, or until a toothpick tester comes out clean. Let cool in the pans for 5 minutes before turning onto a wire rack to cool completely. Level the cakes evenly using a cake leveler or large serrated knife. Cut each cake layer horizontally (torte) so that you have four thin cake layers.
Make the elderflower syrup: Combine the water and sugar in a large heat-proof measure with a pour spout. Heat in the microwave at 1 minute intervals, stirring intermittently, until the sugar is melted and mixture is clear (about 2 minutes total). Let cool slightly and stir in the elderflower liqueur. Apply the syrup to the cooled cake layers using a pastry brush until the cake is well-saturated but not soggy (you may not have to use all of the syrup). Allow the cakes to stand, uncovered, for 30 minutes while the syrup infuses the layers.
In the bowl of a standing mixer fitted with the whip attachment, beat the butter until creamy. Add the powdered sugar and mix until crumbly. Add the lemon juice and beat again. Add milk or cream 1 tablespoon at a time while beating on medium speed, until the mixture is fluffy and spreadable. Add the lemon zest and mix again. Scrape down the sides of the bowl and fold together any unincorporated pockets of sugar. Remove half of the icing to a large bowl and cover with a paper towel to prevent the buttercream from crusting.
Add drops of lemon yellow food color to the buttercream remaining in the mixing bowl. Beat until a consistent bright yellow color is achieved. Transfer the yellow buttercream to a piping bag and snip the end. Evenly pipe buttercream onto three of the cake layers and stack them, ending with the unfrosted layer on top. Transfer cake to a plate or cake stand.
Apply a thin coat of leftover yellow buttercream over the entire cake (crumb coat). Refrigerate until firm. Apply a thick covering of white frosting over the top and sides of the cake, smoothing the surface with an offset spatula. Refrigerate until the buttercream is firm, about 2 hours.
Cover flower stems with plastic wrap. Decorate the top of the cake with flowers and greenery, inserting the covered stems into the cake. Garnish with fresh berries. Using a pastry brush, cover the berries with melted jelly.