Preheat the oven to 350F. Butter and flour three 6-inch cake pans. Add on damp baking strips. Zest and juice a lemon.
In a standing mixer, sift in the flour, baking powder, salt and sugar. Give it a whisk and set aside.
In a large bowl, add egg whites, milk, elderflower syrup, lemon juice, lemon zest and sour cream. Whisk together.
Add a paddle attachment to the stand mixer. Add in room temperature butter in cubes or pieces.
Switch to a whisk attachment. On medium speed, beat for ½ of the wet mixture into the crumby dry mixture. Scrape the bowl down and add the remaining wet mixture. Beat until combined.
Pour the batter into the pans. I use a kitchen scale for more precision.
Bake for 33 - 36 minutes and/or until a toothpick inserted comes out clean.
Let the cake layers cool in their pans for about 5 minutes, them dump them out onto a wire rack to cool to room temperature.