Separate the eggs.

Mix the yolks, sugar, and vanilla sugar until the mixture is white and fluffy.

Grate the lemon zest over the mixture and press the lemon juice into it.

Add the creme fraiche to the mixture and stir to combine.

Whip the cream until thick.

Whisk the egg whites until stiff peaks form.

Gently fold the whipped egg whites into the creme fraiche mixture.

Pound the digestive biscuits, 3 at a time, into crumbs.

Spread half of the biscuit crumbs evenly on the bottom of a cake form with a loose outer brim.

Pour the cream mixture over the biscuit base.

Sprinkle the remaining biscuit crumbs on top of the cake.

Freeze the cake for at least 4 hours until it has hardened enough.