Beat the egg yolks, sugar and 1 vanillin sugar until creamy. Stir in buttermilk and lemon peel. Whip 500 g cream until stiff, stir into the egg mixture (a liquid mass is formed). Cover and place in the freezer for 1 1/2-2 hours until it is thick but still soft. Stir every 30 minutes.
In the meantime, select the berries, wash them if necessary and drain them well. Crumble the biscuits roughly. Add berries and biscuit crumbs, up to 1-2 tbsp. each, to the buttermilk mixture. Spread into a silicone or metal ring cake tin (22 cm Ø; approx. 2.5 l capacity) lined with cling film. Cover with foil and freeze overnight
Whip 100 g cream and 1 vanillin sugar until stiff. Crumble the rest of the biscuits finer. Defrost the ring cake in the silicone mould for about 15 minutes or dip the metal mould briefly in hot water up to the edge of the mould. Turn out of the mould. Decorate with cream, rest of berries and biscuits