14 large egg whites (1 3/4 cups), room temperature
1 tablespoon warm water
1/2 teaspoon salt
1 1/2 teaspoons cream of tartar
2 teaspoons pure vanilla extract
Make the elderflower syrup: In a heavy saucepan, combine water and sugar and bring to a simmer over high heat, stirring occasionally until simple syrup is dissolved. Remove from heat and set aside.
Remove the elderflower heads from their stems, discarding stems and placing heads in a large heat-safe bowl. Add lemon skin and juice to bowl and pour simple syrup into bowl, stirring to combine.
Cover the bowl with plastic wrap or a towel and allow to sit at room temperature for at least 2 days and up to 4 days. Strain syrup through a cheesecloth-lined sieve and discard solids. Syrup can be stored in an airtight container for several weeks.
Preheat the oven to 350 degrees. With a fine sieve, sift together flour and 3/4 cups sugar four times.
In the bowl of a stand mixer on medium speed, beat together egg whites and water until foamy. Add salt, cream of tartar, and vanilla; beat until soft peaks form. Increase speed to medium-high and sprinkle in remaining sugar, 1 tablespoon at a time. Beat until stiff but not dry.
Transfer to a large bowl. In six additions, sift dry ingredients over meringue, folding in quickly but gently.
Pour batter into an ungreased 10-inch tube pan with removable bottom. Smooth top with an offset spatula. Run a knife through batter to release air bubbles. Bake for 35 to 40 minutes, until golden brown and springy to touch.
Invert pan on its legs or over the neck of a glass bottle and let cool completely, about 1 hour. Carefully run a long offset spatula or knife around the inner and outer perimeter of the pan to release cake. Place on a plate, bottom side up; cover with plastic wrap until ready to use.
Serve each slice with some elderflower syrup poured around the base and whipped cream and berries on top.
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