Use a double boiler. Heat eggs, sugar, salt, lemon juice and zest for about 8-10 minutes. Use the spoon test to make sure the mixture is thick! Then, remove from heat and whisk in butter. Do a tablespoon at a time, letting it incorporate before adding more. Then cover chill for 1 hour.

In a bowl, sift together flour, baking powder, and salt. Set aside.

In a separate bowl, whisk together egg whites, milk, and Elderflower liqueur. 

In the bowl of your stand mixer, cream butter and sugar until fluffy, about 4 minutes. Then, Do the rest by hand. Add in half of milk mixture and incorporate. Then, add in half of flour ingredients, folding it in so you don't over-mix. Then repeat. Spread evenly into round buttered cake pans. Bake for 25-30 minutes at 350 degrees. Insert toothpick to make sure it is still moist but is cooked. Let cool completely!

In your double boiler, dissolve your granulated sugar with egg whites and whisk frequently until it reaches around 160 -165 degrees. Remove and place into the bowl of your stand mixer.

With your stand mixer and the whisk attachment, whip egg whites until glossy and stiff.

Then, add in chunks of your butter a couple of tablespoons at a time. Let each incorporate before adding more. At last butter incorporation, sprinkle in salt.

Then add in your Elderflower Liquer.

To assemble as a layer cake:

Cut each round cake in half, lengthwise, with a serrated knife. then, have 4 layers ready to go. On bottom layer, spread buttercream. Pipe an outer "edge" to hold in lemon curd and anchor the next cake layer.

Then, spread half of lemon curd. Top with a cake layer and repeat. 

Decorate the top as desired.