Sift icing sugar into stand mixer with cubed butter and beat until smooth
Add egg yolks and beat until incorporated
Add salt and flour and beat on low until just combined into a loose, crumbly dough
Divide dough into two parts and roll each out between two pieces of parchment until ⅛ - ¼ inch thick
Chill dough for 20 minutes
Preheat oven to 425°F
Line tart tin with dough, add aluminum foil and pie weights
Blind bake crust at 425°F for 10 minutes, then remove weights and foil
Let crust cool and reduce oven temperature to 375°F
Soak elderflowers in heavy cream for 10 minutes
Push cheese through a strainer with the back of a spoon
Combine cheese, breadcrumbs and elderflower-cream mixture in stand mixer
Add sugar and beat until smooth
Beat egg whites until stiff peaks form
Fold egg whites into cheese mixture gently until no white streaks remain
Pour mixture into cooled pastry crust
Bake at 375°F for 50 minutes or until just set in the middle with a slight wobble
Turn off heat and allow to cool in oven with door open for 15 minutes
Let finish cooling to room temperature
Serve plain or garnish with honey or fresh berries
