Preheat the oven to 350F. Line the bottom of two 6-inch cake pans with parchment but do not butter the sides.
1. Melt the butter on low heat and keep slightly warm. Sift flour and cornstarch together in a small bowl.
2. Combine eggs, sugar, and salt in the bowl of your stand mixer and place in a pot over an inch or two of simmering water. You do not want the bottom of the bowl to touch the water. Whisk occasionally until hot to the touch, about 6-10 minutes.
3. Place the bowl with the warm egg mixture onto your stand mixer with the whisk attachment and whisk on medium-high until mixture is pale, has tripled in volume, and has reached "ribbon" stage, 3-5 minutes.
4. Carefully fold your flour into this mix with a spatula in three additions, making sure to get all the flour that falls to the bottom without deflating the batter much.
5. Take about a cup of batter out and mix with the melted butter. Gently fold the butter mix into the rest.
6. Divide evenly amongst your prepared pans. Bake until cakes are golden brown and spring back when you press them with your finger, about 23-30 minutes. Allow to cool completely in their pans before removing.
Meanwhile, make the syrup and compote.
1. Pour your sugar in a small saucepan and cover with water; stir lightly. Bring to boil and allow to boil for a few minutes, until sugar is dissolved.
2. Place an elderflower umbel in a small glass container. Pour the hot syrup over the flower and allow to steep as the syrup cools.
Place all compote ingredients in a pot. Bring to a boil, turn heat down slightly, and simmer for about 20 minutes, or until most of the blueberries have popped and sauce has thickened. Taste and adjust sugar or lemon if necessary.
Once cakes have completely cooled, whip the heavy cream with the sugar and elderflower flavoring until it comes just to soft peaks.
1. Slice each cake into two layers horizontally. Brush each layer liberally with the syrup.
2. Fill layers first with blueberry compote, then a nice layer of whipped cream.
3. Frost with a thin layer of whip to crumb coat; chill in the fridge for about an hour.
4. Finish frosting with the remaining whipped cream. Decorate with fresh blueberries and fresh and candied elderflowers and serve with any remaining compote.