Wash the lemon hot, dry and finely grate the peel. Squeeze the lemon. Mix juice and 1⁄2 tbsp. icing sugar. Sort the berries, wash if necessary and let them drip off. Stir buttermilk with cream cheese and 3 tbsp. icing sugar until smooth. Fold in half the lemon zest and 2⁄3 blueberries.
Put the cantuccini in the freezer bag, seal it and crumble it roughly with a rolling pin. Put crumbs in a form (18 x 25 cm) and sprinkle with sugar-lemon juice. Spread cream on top. Chill tiramisu for about 1 hour.
Dust Tiramisu with cocoa, sprinkle with the rest of lemon peel and other berries.