Use a 28-cm fluted flan tin. Grease tin. Preheat oven to 160°C. Place butter and sugar in a bowl and beat until light and fluffy. Stir in the milk and half the lemon zest. Add sour cream by the tablespoon; stir gently to combine.
Combine the flour and baking powder. Stir into sour cream mixture until thoroughly combined. Transfer mixture to the tin to cover base and side; smooth surface with a knife. Bake base for 40 minutes or until a skewer inserted in the centre comes out clean. Remove from oven and leave in the tin for 10 minutes. Turn out onto a wire rack to cool.
For the filling, pour the gelatine over 60 ml cold water; leave for 5 minutes. Place milk, sugar, vanilla and remaining lemon zest in a pan; bring to a boil. Remove from heat.
Stir the gelatine into the hot milk mixture. Place the water and ice cubes in a bowl. Stand pan in it and stir mixture constantly as it cools, until it begins to thicken.
Whip cream until stiff. Wash and trim berries and pat dry. Stir whipped cream into the cooled vanilla crème mixture and spoon over the flan base. Refrigerate until set. Arrange mixed berries on top and serve.
To add a special-occasion touch, pipe rosettes of whipped cream on the top of the flan, to decorate. Use a mixture of seasonal berries of your choice, or use a selection of frozen berries that have been completely defrosted and drained of any excess juice.